Fixed the shebang line for Arch Linux

On some Linux distros (most notably Arch), python is a symlink to
python3. Since gitall actually requires python2, it did not work
out-of-the-box on those distros. Specifying python2 on the shebang line
should work everywhere. See PEP394 for details.
Add install instructions for Mac OS X
Merge pull request #4 from Grimy/arch-compat

Fix the shebang line for Arch Linux