Add Python 3 support for pygments.rb

This PR adds fully support for Python 3, which could solve all those annoying Python 3-related issues (especially on a distro where the default `python` is 3.x, like Gentoo or Arch. after tired of the temporary `#!/usr/bin/env python2` hack.):
#45, #49, #58, #59

[mentos.py](https://github.com/soimort/pygments.rb/blob/add_py3k_support/lib/pygments/mentos.py) is now made runnable under both Python 2 and 3.

On a system where the default `python` is 2.x, [vendor/pygments-main](https://github.com/soimort/pygments.rb/tree/add_py3k_support/vendor/pygments-main) will be bundled unchanged as before. When the default `python` is 3.x, the code in [vendor/pygments-main](https://github.com/soimort/pygments.rb/tree/add_py3k_support/vendor/pygments-main) will be run through `2to3` after gem installed (just as the installation of Python 3  Pygments did), using [ext/extconf.rb](https://github.com/soimort/pygments.rb/blob/add_py3k_support/ext/extconf.rb).

The only thing I didn't port is the test part, due to the original vendor code is Python 2 only. There's no way to test it on Python 3 without running it through `2to3`.

To test it on a Python 3-default system, build and install the gem first, then change a line in `popen.rb` to something like: (for using the `2to3`-ported vendor code)

```py
ENV['PYGMENTS_PATH'] = '/home/user/.rvm/gems/ruby-2.0.0-p0/gems/pygments.rb-x.y.z/'
```

then all tests shall pass.