Add queueing of activities, starting with harvests

This changeset adds a general queueing mechanism and a specific
implementation of harvest queueing, using Resque.

The tests in spec/models and the comment in Krikri::SoftwareAgent
describe how harvest jobs are put onto the queue and run.

- add Resque, with its dependence on Redis
- Create Krikri::Job, which is extended by particular types of jobs.
- Add Resque console, mounted on top of Krikri's path, on ./resque.
- Update Activity model
- Add queueing-related methods to SoftwareAgent
- Add Timecop dependency for testing.
Merge pull request #35 from dpla/feature/queueing

Add initial queueing mechanism