Correct spelling of "triple"

The word "triple" was misspelled as "tripple" in a few places.
Remove PEP-257 errors

The script has a number of PEP-257 violations which are being used
to demonstrate/test its functionality.  However this means it's
difficult to deploy the script in an environment where all Python
files, including the script itself, will be automatically checked
for PEP-257 compliance.

The errors are removed and will be added as separate tests.
Fix spelling error

In the method `check_blank_after_last_paragraph`, the word
`paragraph` was misspelled as `paragtaph`.
Add more test cases

More test cases are added to replace the test functionality that
was removed from the main module in commit 3773da5.

Also fixed a couple of PEP-8 warnings:  comparison to None should
be done with "is None" rather than "== None".
Prepare first PyPI release.
Add pep257.check_files function.
Merge branch 'fix-pep257-errors' of https://github.com/dpursehouse/pep257 into fix-pep257-errors
Fix docstring of check_files function.
test_pep257 function for testing source code compliance.
Minor stylistic changes.
Add missing tests + minor test changes.