Add option to specify key in TimeStamper processor

[leplatrem]

In my environment, the logging standard is to store the human-readable timestamp in a 'time' entry :)

[coveralls]

Coverage remained the same at 100.0% when pulling ccfec64 on leplatrem:specify_key_in_timestamper into d6dfd8f on hynek:master.

[hynek]

I don’t think ”Property” is the right term here. Maybe something like “Target key in event_dict for added timestamps.”?

[hynek]

Please:

• …fix the nit I pointed out above,
• …add a test doc string a la https://plus.google.com/+JonathanLange/posts/YA3ThKWhSAj
• …add a changelog entry.

Thanks!

[coveralls]

Coverage remained the same at 100.0% when pulling d0ce8c0 on leplatrem:specify_key_in_timestamper into d6dfd8f on hynek:master.

[hynek]

Thanks! I was about to complain about <unreleased> but it actually works well. I need to redo the changelog ASAP. :-/