Migrate to argparse

This branch follows the [Upgrading optparse code](http://docs.python.org/2/library/argparse.html#upgrading-optparse-code) directives.

Users with Python 2.6 should install `argparse` using `pip` (`pip install argparse`). Python 2.7+ already include `argparse`. My approach here is to install argparse as a dependency (which will be already resolved in greater Python versions).

`argparse.RawDescriptionHelpFormatter` allows the `description` and `epilogue` to have their own format (to not being printed in one line). 

The `epilogue` will print `fabric`'s available commands.