Experiment with a new module entry point

This commit introduce structure.read which is a wrapper around
guess_format, read_gro, and read_pdb. This function aims at being a more
intelligent entry point for the splitleafs module.

The read function is now used in splitleafs.py. Hence, the format
guessing is moved out of the main function. If the main function is
still too long (see issue #10), it is reduced a bit.
Remove unneeded import of user.isfile
Move the stat display from main to output

The stat display was the last part of non-cli logics in the main
function. It now exists as a function in the output module, which makes
more sense.

There is nothing left in the main function other than running the basic
functions and handle the exception they can raise. Even if the main
function still looks long, I think I can close issue #10.

Close #10.
Replace splitleafs.py content by libsplitleafs

libsplitleafs contained only the logic for the command line interface.
As splitleafs.py is supposed to be the command line tool, it makes no
sens to have the cli logic elsewhere.