List dependencies in a separate file

List all python module dependencies in `requirements.txt`. Update `README.rst`
and `virtualenv-setup.sh` to reflect the change.

This change should make it easier to integrate gurps-manager with Travis CI. It
also deduplicates critical information.
Add config file for Travis CI. Update readme.

The file `.travis.yml` includes directions for unit testing the application.
Update the readme file with a Travis CI badge displaying the current build
status.
Tweak wording in readme

Make readme explicitly state that GURPS Manager supports the 3rd edition
ruleset.