Make Xautoload a soft dependency

Hi,

The problem:
I'm managing dependencies and class autoloading using Composer Manager. So adding a 2nd autoloader is not required.

Proposed solution:
Add a composer.json to the project, indicating the PSR-4 root, and remove the dependency to Xautoload.
The project page should be updated to mention the possibility to use various autoloaders.

Patch attached implementing the proposed solution.

PS: I had to roll the patch against 2.0-alpha1 instead of 2.0-dev because the dev version is older than the alpha one...