Added contributing informations

It's base on the [Symfony contributing standard][0].

[0]: http://symfony.com/doc/current/contributing/code/patches.html
Improve English in contributor guidelines
Merge pull request #48 from alquerci/add/contributing

Added contributing informations
Removed the convertion that a file should contain only one class

On Python a file is a module and a module is the unit of reuse.
Added interface specification to the coding standard
Improved the Pyhtondoc convention
Improved the coding standard example
Removed the CS for define exceptions out of the dedicated module
Improved coding standards