Skip tests if virtual environment is not active
Add tox configuration
Do not exit shell when in active-dev-env
Ignore tox environment files in coverage
Add tox usage instructions
Add requirement files to manifest template
Add travis-ci configuration.

(crobinso: this is only relevant for github mirrors, but I commit this
 now in the hope that it will motivate me to move to github sooner
 rather than later.)
Allow pip to manage dependencies

- add get_requirements helper method to parse *requirements.txt files
- add install_requires and test_requires options to setup command, this
  are ignored with a warning when using the install command
Add long_description into setup.py