Add basic tab completion using argcomplete

You just have to add eval "$(register-python-argcomplete conda)" to your
profile for this to work. You also have to install argcomplete, but there is a
recipe for it at
https://github.com/ContinuumIO/conda-recipes/tree/master/argcomplete.

More work will need to be done to tab complete packages.

Conda still works fine without argcomplete installed; it just won't have any
tab completion.
Merge branch 'master' of github.com:ContinuumIO/conda