Add extern fetching, source maps and test compilation; restructure targets.

This CL adds curling externs files to the Makefile, generates source maps for
the compiled javascript and Closure compiles the tests. It also restructures
the Makefile targets to debug, release and test.

debug provides a source map, release just compiles the app and test compiles
the test files with a source map.

This CL is derived from https://codereview.appspot.com/34670043.

R=mgiuca@chromium.org

Review URL: https://codereview.appspot.com/36390043