Minimize classpath pants passes to java for running tests

Review Request #204 — Created April 10, 2014 and submitted — Latest diff uploaded

jsirois, tejal
This change decreases the size of the classpath pants passes to java for running tests. It is necessary because this classpath can grow very large, and exceed the maximum argument length on some environments. 

Really the best place for this change was in
But the proper solution would involve changing self._command to a function that returned a context manager (since a temporary jar would need to be created to be used for the new classpath) and this would require refactoring a considerable amount of code.
This fix is also a bandaid fix, as it does not ultimately solve the classpath length problem (it only removes jars from the classpath).
Because of the fact that this fix is hopefully temporary, and a refactor now would require another refactor when a better solution is implemented, I instead opted to make the classpath change in the client code rather than in the lower level java executor code.

I did not add tests/logging because I did not find any existing tests/logging in the source (it would also probably require heavy mocking). If you think the code needs some please let me know and I'll add it.
Manual Testing Done:
./pants src/python/twitter/pants && ./dist/pants.pex goal test tests/java/com/twitter/common:all -ldebug
./pants src/python/twitter/pants && ./dist/pants.pex goal test tests/java/com/twitter/common/objectsize -ldebug
Traced through code to make sure no jars on the classpath/only jars on the classpath would work