Introduced Tox, Travis and CONTRIBUTING

Refs #2, refs #3, refs #4.
In this pull-request:

* added tox.ini file
* added Makefile as a "reference card for usual actions in development environment". Typical use here is `make develop` then `make test`.
* added configuration for TravisCI.org: runs Tox.
* added CONTRIBUTING.rst: content is a proposal borrowed from my own projects... adapt it to your habits as a maintainer!