add the 'lib' directory to the load path

The intent behind this move would be to eliminate
the need to `rake install` the gem as by default it
attempts to load the `git_statistics` gem rather than
the file in the path.

Before:
$ git_statistics
.... some error
.... fix error
$ rake install
.... install gem again
$ git_statistics
.... it works!

After:
$ git_statistics
.... some error
.... fix error
$ git_statistics
.... it works!
Merge pull request #36 from kevinjalbert/load-file-system-not-gem

Add the 'lib' directory to the load path