Add support for header classes via {.class}
First pass at combining id and class attributes in a single curly brace set
Changes to allow both header id and classes in same curly brace set, but only in atx-style headers
Forgot to get rid of an echo
Adds header classes to setext-style headers, fixed regex issues so spaces are treated correctly
Full support for header id & class attributes in the same set of curly braces.
Full support for header classes and id in same set of curly braces.
XML-style empty tags were not recognized correctly when the / was not preceded by whitespace. Also made the regex a little more strict, so that </br/> wouldn't be seen as a valid tag.
Merge branch 'master' into extra

Conflicts:
	PHP Markdown Extra Readme.text
Added multiple footnote backlinks when footnotes have multiple references.
Adding support for class attribute in headers.

Merge remote-tracking branch 'mmcclimon/extra' into extra

is necessary,