Add UMD support

[JSteunou]

Hi,

I started a build umd branch to add AMD / CJS / Global support

Build is ok, have to add minify & clean up the src file to remove previous export and return Stomp object.

Also I was wondering if I could replace lib/stomp.js by the umd build instead of having to files, stomp.js & stomp.umd.js.

[JSteunou]

Done.

All working well, builds are ok, tests are ok

2 more things that can be done

• build src file indentation is a little messed up because of the wrapper
• cake tasks are launched immediately not waiting the end of the previous one (not broken since this PR, just working on this PR make me realise that), so cake build sometimes needs to be launch twice if you start with empty lib/ folder, because lib/stomp.js does not exist yet for the min or test task.

[JSteunou]

still around @jmesnil ?

[JSteunou]

I know you dont have much time but I'll really appreciate a review / merge from you @jmesnil ;)