sort charms by deploy priority before deploying
Sort charms based on class 'deploy-priority' property. Refactor charm importing.

- Import charm classes once at top of _process and sort by deploy-priority class variable.
- Use standard python importlib and pkgutil modules to import classes,
and use explicit __charm_class__ variable in charm modules to avoid
assuming that the first class in the module is the charm.
- Remove now-unused import helper functions in utils.
Merge pull request #11 from mikemccracken/add-charm-priority

Sort charms based on deploy-priority set in charm class. Refactor charm importing.