Adds FunctionFitter class

This can perform minimisation with both chisq and
cash likelihood estimators

Squashed commit of the following:

commit 766ab57c1f69253c8870317a805d629e143816e9
Author: Simon Walker <s.r.walker101@googlemail.com>
Date:   Wed Sep 11 16:30:51 2013 +0100

    Adds documentation

commit c0f0ed9e7f16b054340c866d8fe16f580d46016c
Author: Simon Walker <s.r.walker101@googlemail.com>
Date:   Wed Sep 11 16:27:58 2013 +0100

    Adds function fitter class