JCBC-396: Rename getDesignDocument to align with other methods.

This changeset renames the getDesignDocument and async* to
just getDesignDoc. This helps with consistent naming for all
design document operation.

The old method is marked as deprecated.

Change-Id: I3a2b0df7138404e24619a2ebdbdebca3c74f2412
Reviewed-on: http://review.couchbase.org/31668
Reviewed-by: Matt Ingenthron <matt@couchbase.com>
Tested-by: Michael Nitschinger <michael.nitschinger@couchbase.com>