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:**
```
$ ./bin/git_statistics
.... some error
.... fix error
$ rake install
.... install gem again
$ ./bin/git_statistics
.... it works!
```

**After:**
```
$ ./bin/git_statistics
.... some error
.... fix error
$ ./bin/git_statistics
.... it works!
```