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
Fix some identation
Avoid escaping on HTML comments

HTML comments opening and closing tags were previously escaped. This
wasn't intended.

Fix issue #268
Merge pull request #272 from vmg/fix-escaped-comments

Avoid escaping on HTML comments
Remove Ruby 1.8 from the travis configuration file
Remove encoding conditionals that are no longer required

These are no longer needed since we don't support Ruby 1.8.x anymore.
Forgot about ree in the travis configuration
Directly rely on rb_enc_str_new

Since redcarpet_str_new rely on rb_enc_str_new and this function as
the same prototype under Ruby 1.9.x and 2.x, we can simply remove
this macro from the code.
Remove tests related to the encoding

It's not Redcarpet's job to check whether the strings' encoding is
correctly kept or not. The call to rb_enc_get(text) ensure we keep
the string encoding.
Update Nokogiri to 1.6.0
Merge branch 'master' into drop-1-8
Add changelog entry for 1.8.x removal
Merge pull request #273 from vmg/drop-1-8

Remove ruby 1.8.x support
Update the README file

Add a quick note about the version requirement for Ruby and fix some
typos and grammar.
add additional footnote tests
Merge pull request #1 from sbl/footnotes

add additional footnote tests