Merge pull request #29 from dpursehouse/issue-26-print-errors-to-stderr

Print error messages to stderr
Fix "First line should be in imperative mood" warnings

When pep257.py is run on itself, warnings about imperative mood
are reported on the docstrings of a few of the test methods.

These are false-positive warnings.  Rather than attempting to
update the check to catch all the edge cases like these, modify
the docstrings slightly to avoid the warnings.

Fixes #25

Change-Id: Ic0084f9bdb881de83ab77e516f0bb9195882c359