Add build script.

This script allows us to build RPMs in an automated way.
Merge pull request #6 from mozilla/add_build_script

Add build script.
build: use a human readable date.
build: add "build env" which creates our venv.
travis: Use build env dev to install.
Merge branch 'master' of git://github.com/mozilla/apk-signer
Saner storage logic for keys

- Use a different bucket for key storage
Document the signing configuration options

- Document the various options