Don't override design_doc_id of the view, if its set.
Enable storing function types inside the json body.
IDatabaseClient methods operating on the documents can now work both with implementation of IDocument and pure dictionaries.
WIP on handling conflicts.

* Define various conflict resolution strategies.
* For "migrate" strategy save the log document from update_document()
  routine.
* Define initiation procedure of the _replicator database.
  Check couchdb version and save the design document if needed.
* Test case using 2 databases and triggering conflict states.
Fix unserialization of function type in json.
Extend IDatabaseClient interface.

* Now also can save the document passing dictionary.
* Posibility to pass extra options to get_document(), to get it in raw
  format or with conflicts info.
Implement method merging update logs to resolve conflicts.
Expose a method for the agent to get the agency config.
Typo fix.
Introduce local couchdb document holding partition tag.

This tag is appended to each update log, so that the cleanup logic
can analyze it.
Implement cleanup of update logs.

* Introduce database partition tag saved along the update log.
* Simple test scenario including solving migrated document and cleanup up.
* Implement the replication filter to be used for replication.

TODO:
* Make the scenario more complicated (modify the document in both partition)
between the modifications.
* Test continuous replication.
Refactor time.wait_for() to make it usefull outside the test.
Refactor testcase for replication and cleanup.

This is a preparation to make its scenario more complex.
Implement and test solving subsequent conflicts.
Simplify the return values of configure_replicator_database().
Define filter to get notified about conflicts via _changes feed.
Factor out the method to get details of configured replications.
Add integrity agent.

* Listens on _changes feed and triggers resolving conflicts.
* Run recurring task cleaning up the update logs.
Add tests for integrity agent. Fix some minor issues.
Add api section for integrity agent showing the state of configured
replications.