Move path pointing to propel to Composer's autoload

This one was easy!
Update the contribution guideline

Since the new repository's design, the process to create a pull request
has been a little bit change ; update the documentation to match the
actual one.

Also remove the contribution part in the README file and move it to the
contributing file. The README was up to date with the new testing
process.

Also update the part about fixing checkstyle. To keep clean logs and
make git blame easier, we should only fix files which have been changed.
Update svn co url since svn.github.com is out of date