Make command-line friendly

I mainly just did this as an exercise to try out [argparse](http://docs.python.org/2/howto/argparse.html), but here's how I would probably tackle something like this. Feel free to accept the PR or not, I won't feel offended either way :)

The changes:

* Added a shebang and adjusted the file permissions – this means I can run the script by itself without the `python` prefix, making it easy to install somewhere in my $PATH
* Adopt argparse for options – it's checks for types and is self-documenting (`--help`), so I can call it on a whim without having to remember anything

I'm in the midst of rebuilding my OS X development environment, so I haven't tested this with the unirest library yet, just the argparse stuff. I also don't have an imagelift account, so make sure you check it works as expected before merging.