Swig: add file to generate documentation

* Use swig feature to generate python auto-documentation.
* Still no way to generate javadoc from C++ documentation.
Swig: fix binding Rational issue

* Add avRational.i, which declares AVRational struct to swig.
* Update pyTest which use Rational.
Merge pull request #122 from cchampet/fix_binding_ratio_v2

Swig binding: fix Rational