Added support for use case in which user submits single training and testing files via train_file and test_file in the configuration file.
Removed check for whether test_file is set when checking set-up for 'evaluate' and 'cross_validate' conditions; since test_file is set to test_path earlier, this check can be removed.
Removed parens around (testpath) in two if statements for improved readability.
Removed the following line, since the test will never be false:

if config.has_option("Input", "featuresets"):
Merge branch 'master' into feature/train-test-filename

Conflicts:
	skll/experiments.py
Tweak handling of single file feature sets.

- Now create proper feature set names with names of train/test files.
- Just use isfile to see if we're dealing with a file or a directory.
Fix missing import of isfile in experiments.py
Merge branch 'master' into feature/train-test-filename
Add a simple test for loading a single feature file w/ _load_featureset

Also do a bunch of PEP8/PEP257 cleanup.
Add tests for train_file and test_file

In addition:

- Remove old cruft about vocabs dir from some directory
- Add check to make sure test_file and test_location aren't both set
Merge pull request #197 from EducationalTestingService/feature/train-test-filename

Feature/train test filename