Align td and th with css instead of align property

When you align tables with css, <td align="..."> gets overridden.
Using inline styles keeps alignment no matter what CSS contains.
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
Redcarpet v3.0.0
Output <hr/> when using XHTML renderer
Add basic assertions for the HTML_TOC render

Since we don't have any tests for the render object, we should ensure
that a minimum set of assertions are passing before trying to improve it
with additional stuff.
Merge pull request #271 from brief/footnotes locally

Add PHP-Markdown style footnotes support

Conflicts:
	test/html_render_test.rb
Wrap some parts of the README to 80 chars [ci skip]
Add a mention about Redcarpet 2.3.0 for 1.8.7 users

In order to avoid issues like #280.
Fix Prior to -> Starting with in the README

Thanks @parkr!
Specify the minimum required Ruby version (1.9.2)
Add callbacks from f2ad99f to the README [ci skip]

Add the following callbacks to the README:

  - footnotes(content)
  - footnote_def(content, number)
  - footnote_ref(number)
Allow to set a maximum rendering level for HTML_TOC

Allow the user to pass a `max_level` option when instantiating a new
HTML_TOC render object in order to limit the nesting level in the
generated table of content.
Adding space
Merge pull request #279 from vmg/html-toc-render

Allow to pass a nesting_level option for HTML_TOC