Add support for footnotes

Courtesy of @bdolman and @adamflorin
Fix footnote parsing

Consecutive footnotes do not need to be surrounded by blank lines,
courtesy of @microjo. For example, this is now possible:

[^1]: footnote1
[^2]: footnote2
Add test for footnotes
Edit readme and changelog
Added contributors to changelog
add additional footnote tests
Merge pull request #1 from sbl/footnotes

add additional footnote tests
Output <hr/> when using XHTML renderer
Merge pull request #271 from brief/footnotes locally

Add PHP-Markdown style footnotes support

Conflicts:
	test/html_render_test.rb