Feature Addition: generalize EXTENDS for other prefix-groups (e.g., plugins) and add parameters for ipcluster

[clearf]

This commit:

• Reorganizes config.py to generalize extends to other prefixes (to allow, e.g., plugins to extend each other)
  • Add tests to ensure that the use of EXTENDS behaves as expected for a plugin
• Modified the template config to include examples of EXTENDS in a plugin
• As an additional note, this PR does not pass all nosetests, but per (http://star.mit.edu/cluster/mlarchives/2700.html), neither do tests run on a clean develop or master branch.

[cancan101]

This should probably be broken up into two PRs.

[clearf]

OK, I'll split up and rebase. Good point.

[clearf]

@cancan101 OK, I took out the ipcluster changes, thanks.