Make the doc argument optional; use the docstring for the caller's module by default

[kalekundert]

The most common way to use docopt is to write a module docstring and to pass it to docopt.docopt() using the special __doc__ global variable:

import docopt
args = docopt.docopt(__doc__)

This pull request makes this the default behavior. In other words, the code below would be equivalent to the code above:

import docopt
args = docopt.docopt()

There are two advantages to having this be the default behavior. First, users who aren't experienced enough to know that docstrings are accessible to their scripts via __doc__ won't need to worry about that detail anymore. Second, this saves some typing in the most common case. I type the docopt header in almost every script I write, so shortening it by ~20% would be nice.

It's worth noting that this feature uses the inspect module to interrogate the caller's stack frame and global namespace. This is somewhat magical, but I don't think it's any more magical (or different in philosophy) than using sys.argv by default.

New tests and updated documentation are included in the pull request.