(#136) - Fix "Changes limit and filter" against CouchDB 2.0

Do not assume incremental sequence numbers in the test.
(#136) - Fix "changes limit and view instead of filter" against CouchDB 2.0

Do not assume incremental sequence numbers in the test.
This commit also adds a helper function to dig out specific items from
_changes (seems the simplest possible solution rather than importing
lodash or similar).
(#136) - Ignore "changes last seq" against CouchDB 2.0

In clusterd CouchDB, the sequence number returned to the client is an
encoded representation of the various shard states. Whilst the last
"seq" value in _changes.results and the "last_seq" field should evaluate
to the same thing on the server, their encoded representation may differ.
Therefore, the assertions in this test don't hold for clustered CouchDB.
Merge branch '136-changes-limit-and-filter' into 136-master