CSS style inheritance and more

Currently implemented CSS styles are inherited from parent elements (#78) if they should according to this list: http://stackoverflow.com/a/5612360 **(please add new symbols to StyleResolver class>>inheritableAttributes, if needed!)**

Styles are resolved lazily using the StyleResolver (Visitor-like). "Local" style rules are processed all at once and the results are cached in the computedStyles-Dictionary of a DOMNode. If a style is not found in that dictionary and can be inherited, the parent's style is used (which can again trigger the inheritance mechanism). If there's no parent (e.g. for the DocumentNode), nil is returned. The resolved value is added to the local computedStyles dictionary, so that it is cached.

In practice, all local styles are processed when the document is rendered because the value of the display-attribute has to be determined for each element. (This takes a while...)

I've also added 

 - two bugfixes for the new rendering engine
 - real support for the display-CSS attribute (inline, block, inline-block), previously hardcoded into Html*Node classes
 - default styles for some elements (including fixes for #63, #64, #66, #71, #72) according to Firefox' default CSS: http://mxr.mozilla.org/mozilla-central/source/layout/style/html.css
 - tests