The 'Got myself a parser' release.

I adapted pypy's parser so now i can play more freely with the language. Let's see what comes out of this. Meanwhile, enjoy the changelog:

• source() is out. use python's import system.
• Support executing foo.py().
• Much better command detection.
• CommandNotFound exception is now a subclass of NameError.
• Allow Command keywords be named like -l and --long-option, so it supports options with dashes.
• This also means that long-option is no longer passed as --long-option; you have to put the dashes explicitly.
• bash() does not return a single string by default; override with single=True.
• Way more tests.
• Updated docs.