[SPARK-5628] Add version option to spark-ec2

Every proper command line tool should include a `--version` option or something similar.

This PR adds this to `spark-ec2` using the standard functionality provided by `optparse`.

One thing we don't do here is follow the Python convention of setting `__version__`, since it seems awkward given how `spark-ec2` is laid out.

Author: Nicholas Chammas <nicholas.chammas@gmail.com>

Closes #4414 from nchammas/spark-ec2-show-version and squashes the following commits:

914cab5 [Nicholas Chammas] add version info