Added a mock version of Couchnode for easier testing.

The mock version of the client is also designed to work when included
into a browser page.  Added a Makefile directive to use node-browserify
to build a directly browser-usable version of the library.

Change-Id: I2ef9dbc73065656e9dc20a5e882b810a43ea4a0b
Reviewed-on: http://review.couchbase.org/31003
Reviewed-by: Mark Nunberg <mnunberg@haskalah.org>
Tested-by: Brett Lawson <brett19@gmail.com>