FEATURE Experiment.Pulling.fitForceExtension()

Updated curvefit.MMS to use numpy roots to solve for inverse WLC
model. Also some minor bug fixes to Fit class to handle the
inverted nature of this fit.

Added function fitForceExtension() to fit any fitting function
to specified parts of the FEC. Defaults to MS now out of simplicity,
but MMS also works so long as you fix K. Maybe an improvement is
to have K fixed by default in that function?

Updated README to reflect these improvements.
Merge branch 'master' of github.com:cfperez/smbanalyze

Conflicts:
	README.md