Ability to run use salttesting from a directory that is not adjacent to the salt source

[mikn]

Right now you have to put the tests next to the salt source itself, although it might be valuable to be able to write integration tests for custom states outside of salts source directory. Hence it would be neat if you could send in a flag specifying the source directory to put into the python path.
I will supply a pull request with a hack-ish patch to accomplish this!