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

To get the Bounds, the Union method of the rectangle must be called only when the first rectangle has been set. If the Renctangle is empty, use the first children's Bound, as a rectangle.

http://www.w3.org/TR/SVG/pservers.html#RadialGradientElementFXAttribute