Updating existing entries

[mewa]

Right now it's impossible to update already added entries.

Removing them and adding again feels cumbersome, especially if we know our intention.

[pradykaushik]

@robfig pull request #274 ready for review.
Also, as you have marked #273 as a duplicate of this issue, should I update my PR to allow updating other members of Entry as well? The only members that I feel that we should allow updating should be Schedule and Job (if at all). Please let me know.