Fit.plot() uses FRET._subplot; ADD MMS_rip

Also including changes to FRET.plot() that keeps hold() at current
setting unless explicitly changed with keyword argument.
FIXED useful.toInt() to return 0 instead of None
FIXED FEC curve fitting (Issue #5) and improved MMS

Experiment.fitForceExtension() now properly handles too low and
too high ext and force arguments.

Also adds helper functions so that a good Lc is chosen automatically
and K is fixed by default for fits starting with MMS. FOLLOW THIS
CONVENTION for MMS type fit functions.

Changed ordering of Fit() arguments to initializer: fitfunc is now
LAST.

curvefit.Fit().__repr__() improvements to store and print fitfunc
name at console.