Support "none" in Fill value if inherited from parent

If elements are nested in groups and have different fill values than
their containing group the fill value wouldn't be written correctly.

- Changes to get texts displaying bold / normal weight
- Changes to get textspans work correctly
- Made (semi-hardcoded) document metadata section - allow reading and regurgitating metdata tag from an original source document
- Included image
- Modified paths to allow for scientific notation
- Included ability to have markers for path ends (arrow ends etc)

- Fixed support for text-anchor="middle" and text-anchor="end" for <text/> elements.
- Added more XML API documentation
- Added x attribute and y attribute support to <use/> elements.