Added way to retrieve existing jobs per tags.

[Skenvy]

In making extensive use of multiple tags on each Job (that define multiple configuration settings that define what that job does, as well as manufactured guids) I've encountered the following need/nice-to-have.

I'd like to be able to use schedule.get_jobs(some_tag) to retrieve an iterable of Job instances which I can then use to log/report on the other tags that have been attached.

If I have functionality that finds Job's that can be periodically removed by comparing against a table of changing guids, but I've used the tags to hold the relevant information that I would like to log to have a human readable history of what Job's were removed, I need a way to get Job's by tags.

The alternative would be to have the Scheduler.clear(...) return the list of jobs that it had removed.

[coveralls]

Coverage remained the same at 100.0% when pulling 4e42fe3 on Skenvy:master into 90fb86f on dbader:master.

[SijmenHuizenga]

Hi! Thanks for this pr. Good idea to add this! Would you be able to add some tests and an example in the docs? Let me know if you need help with any of that.

[Skenvy]

Added/fixed the test I added for this. Let me know if you need anything else for this PR

[Skenvy]

Squashed the changes into 1