Minuit integration

[pelssers]

This implements integration of the Minuit minimizer via the iminuit package.
Currently 'bestfit_minuit' provides the same functionality as the 'bestfit_scipy' function ('bestfit_scipy' is still the default).
Minuit can be used by setting the 'bestfit_routine' parameter in 'plot_likelihood_ratio' or 'one_parameter_interval' to 'bestfit_minuit'.

This addresses/solves issue #10

[JelleAalbers]

Solid work Bart! I made some small additions:

• Made iminuit the default bestfit routine, if minuit is installed. Otherwise switch to scipy, but berate the user for not installing iminuit :-) I've had some serious headaches with the scipy minimizers in blueice previously, and again a few days ago on a (degenerate) test problem (something with the "dumb" spatial likelihood I think).
• Set defaults to produce no extra output, since bestfit is called iteratively from the higher-level functions in blueice.inference. Do you think a common quiet=True/False switch for both minimizers is a good idea? Since the extra output is very useful for a big final fit and debugging.