Replace sentries with juju run

First pass, replaces existing sentry features with juju-run
equivalents while keeping the same API.

Includes functional tests that prove new juju-run output
matches old sentry output.
Clean up test setup
Replace old sentry code with new.

- Delete all old sentry code and tests
- Split unit/functional test make targets
- Update travis config to only run unit tests
- Fix unit tests that failed due to sentry code removal
Remove unnecessary sentry-related Deployment args.
Remove more dead code

- get_relation() is no longer used
Use series specified by Deployment

- When deploying charms that don't explicitly specify a series,
  default to using the series set on the Deployment (instead of
  always defaulting to precise).
Merge pull request #46 from tvansteenburgh/better-sentries

Reimplementation of sentries using `juju run`