Python 3 support

[ThePeach]

Any plans into migrating the code over to Py3?

[ari3s]

Pull request #61 solves Python 2 and Python 3 compatibility.

[arisliang]

when running ./gitstats, it still shows error:

/usr/bin/env: ‘python2\r’: No such file or directory

[linechronic]

Update gitstats to python3:
#105