Allow save & save! without performing validations

Fixes #17

Change-Id: I2dad010c8739bf7b0e81b02f449877edaa26a16b
Reviewed-on: http://review.couchbase.org/41272
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>