ENH: using python's warnings instead of prints in fitpack.

When fitpack detects a recoverable problem, it will signal it with
a RuntimeWarning instead of printing to stdout. This way the output
can be programatically silenced or made exceptions.
PEP8: putting back print_function for uniformity
DOC: indicating the deprecation of the parameter quiet
Merge pull request #4003 from Dapid/warnings

ENH: using python's warnings instead of prints in fitpack.