Adding "active" property when it is not returned from API.

This happens only when requesting an specific repository. When requesting information about multiple repositories the "active" property is filled properly. The Travis team are already aware of this and soon this should be working as expected.
Adding "duration" property to Job entity.

When such information is not provided by API it is necessary to calculate it based on "started_at" and "finished_at".
Fixing how to calculate Job.duration so it can be supported by Python 2.6.
Improving coverage.
Merge pull request #15 from menegazzo/fb-14-missing-properties

Fixes #14