Use python exec from virtualenv

This is no longer easily testable by unit tests, add an integration test
that can be run to test virtualenvs and externalPylint.

Only now when writing this do I realize that it could have been tested by
unit tests by mocking grunt.util.spawn.. Oh well, that's left as an
excercise to the reader of the git log.
Refactoring
Add jshint options and delint
Merge pull request #7 from thusoy/pythonexec

Use python executable from virtualenv.

Also prompted some refactoring and delinting.