Updated "Server:" response header
Updated "Server:" response header

Header value for unofficial builds should still include a parseable
version number, else CBL gets confused.

Added Git branch name and commit SHA to header value for unofficial
builds.

New header looks like:
Couchbase Sync Gateway/1.2 branch/fix/server_header commit/5bfcf79

Fixes #1209

couchbaselabs/sync_gateway,ceejatec/sync_gateway,couchbaselabs/sync_gateway,tophatch/sync_gateway,paulharter/sync_gateway,paulharter/sync_gateway,tophatch/sync_gateway,tleyden/sync_gateway,ceejatec/sync_gateway,paulharter/sync_gateway,tophatch/sync_gateway,tleyden/sync_gateway,ceejatec/sync_gateway,ceejatec/sync_gateway,couchbaselabs/sync_gateway,tophatch/sync_gateway,tleyden/sync_gateway,couchbaselabs/sync_gateway,tleyden/sync_gateway,paulharter/sync_gateway