.Net 3.5 Support and Rendering Improvements (Markers, TSpans, Paths, etc.)

- Adding a descendants convenience methods (similar to the Xml to Linq
API)
- Removed non-functional property from the SvgDescription class to
encourage use of the Content property instead
- Added unit test project for unit test of new functionality.

fixed up the support for markers so that they draw properly, and added support for "preserveAspectRatio" except for the 'slice' and 'defer' options.

- Fix issues with calculating the next coordinate after a close figure
and a relative move
- Allow the std. deviation in a gaussian blur to be a float.
- Allow "none" for a unit collection
- Allow a marker to scale with either the stroke width or the use a
custom coordinate space
- Allow gradients to reference other gradients defined later in the SVG
- Respect the font and fill styles of TextSpan elements.  Current
support is still partial.
- Fix href attribute bug on the SVG use element

Reading EntityReference on open when use as a node

- Implemented .Net 3.5 support mostly by removing ISvgEventCaller
functionality in .Net 3.5 build
- Added ability to parse orient="auto"
- Added ability to parse colors such as rgb(#%, #%, #%)
- Fixed marker rendering implementation

Read EntityReference node and add it to the SvgDocument.

```
<?xml version="1.0"?>
<svg xmlns="http://www.w3.org/2000/svg" width="800px" height="800px">
  <g>
    <rect width="200px" height="200px" fill="black" />
    <text x="100px" y="100px" font-family="Verdana" font-size="24" fill="white" text-anchor="middle">&name;</text>
  </g>
</svg>
```

Conflicts:
	Samples/SVGViewer/SVGViewer.csproj
	Samples/SVGViewer/SvgViewer.Designer.cs
	Samples/SVGViewer/SvgViewer.cs
	Samples/SVGViewer/SvgViewer.resx
	Samples/SVGViewer/app.config

Support for `"display=none"` #66

Moved Display="none" hack & Added .Render check for : if (!Visible ||
!Displayable)

Also, minor change to logic of SvgBoolConverter so bad visibility values
(other than "hidden" or "collapse") wouldn't accidentally hide element

RGBa support
exponent support in units (IE sucks!)
inherit font-size and font-family from root element on text elements
allow empty text elements if there is a non empty TSPan
added support for "undefined" opacity value (forgot to put this in the commit msg)

- exponent support in units (IE sucks!)
- inherit font-size and font-family from root element on text elements
- allow empty text elements if there is a non empty TSPan

Updated to .Net Framework 4, added some code to finish test, & removed
extraneous files. I updated this project so I could test the changes for
#66

Added trim character on ValidateFontFamily

Added support for dx and dy properties for text. 

Changes

Some programs (like InkScape) use single quotes for the font-family.
I've added the single quote character, to the trim.

Added support for dx and dy properties for text. A bit of plumbing was already in place so I completed that.
http://www.w3.org/TR/SVG11/text.html#TextElementDXAttribute
http://www.w3.org/TR/SVG11/text.html#TextElementDYAttribute

BugFix - Text element with no text rising exception

Fixed Bound Calculation