Create base resource interface

[dnaeon]

Currently Gru is able to send tasks to remote minions, which once they receive the task simply execute a command.

We should take it further and implement a base resource type, which would give us a layer of abstraction on the commands/tasks being executed.

This would allow for creating resources such as package, file, service, etc. which would manage things in idempotent manner.

[dnaeon]

Implemented in resources branch. Will be merged into master as part of the next release of Gru.