Moves from optparse to argparse

[thisisdhaas]

See https://docs.python.org/2.7/library/argparse.html#upgrading-optparse-code for a checklist of things that should have been done (which I tried to follow), and http://stackoverflow.com/questions/3217673/why-use-argparse-rather-than-optparse for a discussion of why this is a good idea in general.

See #26 for a discussion of dropping support for python 2.6 in spark-ec2 2.0

[shivaram]

Thanks @thisisdhaas - Change mostly LGTM. One thing is that it would be good to change the README to note that we require python 2.7 from here on

cc @nchammas

[nchammas]

Didn't test it out, but the changes LGTM.

And yes, the README should make it loud and clear that we've dropped Python 2.6 support. Better would be to capture that in code somehow, so the error when someone runs on 2.6 is clean, but a README note is good enough IMO.