add "gulp watch-build-tests" and organize gulpfile
add "gulp watch-build-tests" and organize gulpfile

This adds a missing piece of the gulp watch abilities for datt. You can now run
"gulp watch-build-tests" and the tests file will automatically be rebuilt.

yoursnetwork/yours-core,dattnetwork/datt,yoursnetwork/yours-core,dattnetwork/datt,ryanxcharles/datt,ryanxcharles/datt