Add maximum likelihood fitting routine

[slwatkins]

I've added a couple of functions that use iminuit to do maximum likelihood fitting for some common spectral shapes:

• flat background
• exponential backgrounds
• gaussian backgrounds

This supports multiple of each background (except flat background, since multiples would be degenerate).

Also added a corresponding demo notebook, which shows how to use the functions. It's a little bit unwieldy at first, but hopefully the demo is helpful.

And, as always, did some PEP8 white space fixes....

Resolves #56.