Added example

Added a new example to the Programming Guide showing how to change the default font and font size.
Options for HTML

Render HTML with options.
Adds support for background CSS shorthand

Currently just pulls out the color property and sets background-color
Merge pull request #726 from thusfresh/backgroundshorthand

Adds support for background CSS shorthand
NSCoding for DTTextBlock and DTCSSListStyle

Includes unit tests and some compatibility methods for NSCoder
Adds NSCoding for DTTextAttachment

Unit tests still need to be written
Merge pull request #728 from thusfresh/nscoding

NSCoding
Merge pull request #717 from depl0y/develop

Minor changes in programming guide and DTAttributedTextCell