Inline CSS and basic font formatting support

This (partially) fixes issues #29 and #33:

 - Styles included in the document are processed and assigned to the corresponding DOM elements (including inheritance)
 - Basic support for "font-style", "font-weight" and "font-size" CSS properties and corresponding "font"-shorthand property
 - The code is unit tested

http://de.selfhtml.org/css/eigenschaften/anzeige/font.htm looks nice.

**Further Todos:**

 - Support for text-decoration:underline and text-align (easy), font-family and line-height (harder)
 - Improve support for absolute font-size values (currently only cm supported - used on selfhtml :smiley:), little effort
 - Proper inheritance of styles: e.g. use parent element's style to process font-size: 200% on child element, medium effort