Support multiple versions native extension gems
Bump version
Linux: link mysql2 extension to libstdc++

I can't find a way to statically link it to libstdc++ and libgcc_s.
However, considering that libstdc++ has been ABI backward compatible
for 9 years[1], it is probably ok to dynamically link to it.
It seems the story with libgcc_s is similar.

Closes GH-21.

[1] http://stackoverflow.com/a/14082540/20816
Revert "Support multiple versions native extension gems"

This reverts commit eab8aec929f17c12ecaabc7ff4b82042c87e2739.

It's difficult to compile Nokogiri 1.5 on OS X.
Add eventmachine and thin native extensions
Bump version to 20141224