Formatters should not stringify datetimes.
Rewrite how and where DocManagers are created.
This provides a cleaner Python API for creating a Connector, as well as shortening the syntax for --doc-managers on the command-line. This introduces some backwards-breaking changes:

- There is no more camel-cased --docManager option. This is superseded by --doc-managers or -d.
- The doc_manager keyword argument has been renamed to doc_managers in the constructors for Connector and OplogThread.
- doc_managers takes a list of DocManager instances, rather than a list of strings.
- Remove the u_key and target_url options from the Connector constructor, as these are now obsolete.
- --doc-managers takes a comma-separated list of DocManager module names, e.g. 'elastic_doc_manager'.
Update CHANGELOG.rst to reflect changes made to --doc-managers.
Update version in setup.py to 2.0.dev0.
Keep namespace and timestamp metadata in a separate Elasticsearch index.
Yield both document and meta-document actions in the same generator for bulk_upsert into Elasticsearch.
Remove redundant _id in meta document for regular upsert using the Elastic DocManager.