rename hit to set

[zouharvi]

Fixes #191.

See justification in the original post. TL;DR, calling something "HIT" in front of real people is confusing. The alternatives are:

• task, but we use that word for something else internally
• job, but that sounds like something that takes much longer(?)
• episode, block, chunk, collection, but somehow not fitting well(?)
• set is not perfect but gets the point across? I.e. set of ~100 annotations.

Which would you prefer?