Include Go build tags for Enterprise edition builds

Hi build team,

We're prepping for SGW 2.5.0, which includes both CE and EE builds. In order to achieve this, we need to pass a build tag into the Go commands when building the enterprise edition of Sync Gateway.

This change is backwards compatible, so building older versions of SG, or SG Accel and specifying the build tag will not change anything. It will just ignore the tag if older code is not set up to recognise it.

I've currently set up a build tag on a branch, called cb_sg_enterprise, there's an example below on how to use it in the build and test steps of sgw_unix_build.sh. The same will need doing for the Windows build script.

For example:
CE: go install github.com/couchbase/sync_gateway
EE: go install -tags cb_sg_enterprise github.com/couchbase/sync_gateway

CE: go test -race github.com/couchbase/sync_gateway
EE: go test -race -tags cb_sg_enterprise github.com/couchbase/sync_gateway

Please let me know if there's anything else required, or if you'd like to change the name of the build tag to be consistent with other Couchbase projects.