Fixes #300. Adds PYTHON2 variable for use in cpplint

[syclik]

Submisison Checklist

• Run unit tests: ./runTests.py test/unit
• Run cpplint: make cpplint
• Declare copyright holder and open-source license: see below

Summary:

Adds a PYTHON2 make variable to control cpplint behavior.

How to Verify:

Run make cpplint PYTHON2=<some other python version>

Side Effects:

Removing the RUN_CPPLINT make variable.

Documentation:

Updated in the top level make help.

Reviewer Suggestions:

Anyone.

Copyright and Licensing

Please list the copyright holder for the work you are submitting (this will be you or your assignee, such as a university or company): Columbia University.

By submitting this pull request, the copyright holder is agreeing to license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)