Generate manpage with sphinx and add it to package

[dadav]

This commit addresses issue #162 which suggested to add a manpage to the tldr python client. The changes in the tldr.py file where necessary, because sphinx needs an easy way to obtain the Argparser-object. I also added a Makefile, so the manpage can easily recreated.

You need the sphinx-build tool and the argparse extension. Both can usually installed via. your package-manager. In my case the packages where called python-sphinx and python-sphinx-argparse.

Closes #162

[dadav]

Sphinx apparently needs the runtime dependencies to be installed. For an easy way to install them and to prevent duplication, I put them in the requirements.txt... If this is not wanted, just tell me :)

[MasterOdin]

Thanks again for your contribution!