add new sections to docs and installation

sections added:

* Contributing
* Credits
pep8
add missing requirements file for contributors
extra environments for tox
use tox environments instead of travix matrix

also use coveralls
add Makefile for developer convenience
add coverage configuration file
use built-in coverage mechanism. ref e89c1c5
revert back to travis matrix and nosetests
Merge pull request #60 from slafs/docimprovement

Coverage, lint and docs