Remove or make optional xmin fitting print

[nialov]

When used as part of a library the xmin fitting prints from powerlaw are somewhat of a nuisance. It would be better to make it optional with e.g. a keyword argument verbose=False|True with which the user could disable the prints.

https://github.com/jeffalstott/powerlaw/blob/6732699d790edbe27c2790bf22c3ef7355d2b07e/powerlaw.py#L230

And as I look through the code I spot that there already exists a verbose flag but it isn't enabled for the xmin prints:

https://github.com/jeffalstott/powerlaw/blob/6732699d790edbe27c2790bf22c3ef7355d2b07e/powerlaw.py#L47-L49