sync_gateway: 1.0-beta2 - couchbase mobile sync

sync_gateway provides the syncing backend for couchbase mobile and is
used together with couchbase-lite-ios or android to develop syncing apps
for iOS or android.

Since the project itself is named sync_gateway I kept the name for the
formula as well.

It uses git for the stable 1.0-beta2 since go needs the submodule references 
to pull in the dependencies during the build, those references are lost when
downloading the zip from github.