clean-up make process:
- downgrade browserify to match es6ify reqs
- peg to specific versions
- set window.Tangram instead of using browserify 'standalone' option (doesn't play together nicely in version of browserify required by es6ify)
Merge pull request #25 from tangram-map/traceur

Traceur
Merge branch 'master' into camera-es6

Conflicts:
	dist/tangram-worker.debug.js
	dist/tangram-worker.min.js
	dist/tangram.debug.js
	dist/tangram.min.js
update ES6 module import style
update build
fix makefile depedencies (need to add es6ify transform to browserify dependency list generation)
Merge branch 'master' into camera-es6
allow focal length to be configured in scene defition, and support interpolated lengths between zoom ranges