Some changes to run script

[eliasdorneles]

Hey, @redapple !

Here are a few small changes for the run_parslepy.py command line tool.

Quick summary:

• added run_parslepy.py to package installation setup (so you can run it from anywhere, provided you have installed parslepy)
• migrated argument handling to argparse
• turned required arguments into positional args, and changed separate arguments for file and url into only one (input)
• updated docs to reflect these changes

I figured it was reasonable to expect the command line tool will probably always require a parselet path and another input (URL or file) that it was justified to make them positional and required.

_{Tip: to test the installation script, I recommend using mktmpenv from virtualenvwrapper.sh, which creates a temp virtualenv and destroys when you do deactivate.}

So, what do you think?