CB-4875 updated readme with info on accessing last supported versions of BBOS and Playbook
CB-4876 removal of playbook implementation
switching version to 3.1.0-dev on master
[CB-4880] release notes for 3.1.0
[CB-4345] Added warning message for when signing keys are not installed

- Added check for bbidtoken.csk
- Added information to link old signing keys to the blackberry id token
- changed check_reqs-utils into signing-utils
- Migrated and added unit tests for signing-utils.js
- switched warning message for when author.p12 DNE to when barsigner.db DNE

Reviewed by Bryan Higgins <bhiggins@blackberry.com>
Tested by Daniel Audino <daudino@blackberry.com>