(#1848) - housekeeping on query() docs

Adds more notes to the db.query()
documentation, especially when we
forgot to mention certain
parameters. Also adds documentation for
options.stale and options.group_level.

The only new code this will require in
order to be accurate is pouchdb/mapreduce#85.
(#1860) - Fix info.doc_count for websql/idb
(#1869) - performance improvements for idb.get

Also includes some generic performance
improvements for merge.traverseRevTree and
utils.isDeleted.
(#1875) - use a library for extend, and then use clone for the 95% of the time where that is what we are using extend for