Reorganized fret.py => fcalc.py and fplot.py

Better represents division of use and removes some dependency
problems that were starting to surface.

Also added fcalc.processMatch() convenience function.

Updated __init__.py, experiment.py, and curvefit.py to reflect changes.
ADDED Pulling.savefig() feature

Saves figures to file using original filename as default.

Constants.FIGURE_TYPE = '.png' is the default filetype.