RF: Simplify entry-points, restore preproc discovery

[effigies]

This places sys.exit in the main entry-point, to reduce the chance of unexpected exits for any external calling code.

This also restores preproc discovery, prioritizing that found in <bids_dir>/derivatives.

Other minor simplifications were also applied, such as allowing argparse to apply os.path.abspath to file inputs.

Closes #32
Closes #34

[effigies]

@adelavega This doesn't need thorough review; just want to make sure that it doesn't break things for you.

[adelavega]

Yep, I know. I just won't get a chance to test that until tomorrow.

[effigies]

Cool. No rush.

[effigies]

I was able to run your models all the way through. I had fmriprep in the output directory, but that shouldn't make a difference at this point. Merging.

[adelavega]

ok cool.