Switch branch, check file is valid. New Git Class.

Code now can switch branches and check if the file is within the given
path.

Also refactored the code to make it more readable. Created a new GitRepository
class in GitRepository.php.

New functions:
* CloneGitRepo will clone a git repository
* FindAndReadFile will look for the file and read its contents. Currently, it
only checks if file is valid. More functionality later.
* GitCheckoutBranch will switch branches

Removed chdir() from code.

Change-Id: I8cb413a1e8abe2e3741f6ef29465cdd156e34a38