use upstream scp module from @jbardin

StarCluster's scp module originally came from:

https://github.com/jbardin/scp.py

before it was packaged nicely on PyPI. All of StarCluster's scp
improvements have been committed upstream (along with a few bug fixes)
and released in the 0.7.0 version of scp on pypi:

http://pypi.python.org/pypi/scp

Removed StarCluster's internal copy of scp in favor of declaring scp as
an official dependency.

closes gh-331
Merge branch 'use-scp-on-pypi' into develop

closes gh-331