Allow the use of checkpoint to be optional

A new parameter use_checkpoints which defaults to be true can be either used in the replicator section of
ini files and/or used in the replication command or doc added to the replicator db. When this option is set
to false no checkpoints are taken. It should be used with caution and only for small dbs.

BugzID:14327
Merge pull request #14 from cloudant/14327-make-checkpoints-optional-2

Allow the use of checkpoint to be optional

BugzID: 14327