add option --dump-render-tree for viewing the WebKit render tree

This can be useful for understanding the structure of the input HTML
and seeing how it translates into the browser's render tree.

This is not available for Qt5 as support for it was removed:
http://trac.webkit.org/changeset/136235/trunk/Source/WebKit/qt/WidgetApi/qwebframe.h
remove support for building with older Visual Studio versions

VS2013 Community Edition can be used for development of open-source
projects for free. In addition, the builds for the older versions
are not being tested (e.g. for compilation errors) so it makes sense
to standardize on VS2013.
remove instructions for building on OS X 10.6

The current releases are built on OS X 10.7 and hence these remain
untested -- it is better to document a process which is being used
regularly and is repeatable.
revert 863774a2 as it is not needed any longer

as VS2008 support was dropped in 5656bb9e
Merge remote-tracking branch 'upstream/master'