Add ability to receive live output from running process

Added new event 'git.process' to permit this feature.

Add GIT_PROCESS constant

Add ability to recieve live output from running command.

> The run() method takes care of the subtle differences between the
> different platforms when executing the command.

> When executing a long running command (like rsync-ing files to a remote
> server), you can give feedback to the end user in real-time by passing
> an anonymous function to the run() method.

Ref: http://symfony.com/doc/2.0/components/process.html