Return browserify bundle instead of generating the bundle

I've made the change and fixed a bunch of tests, but there are some that I couldn't. They are all using run.js in order to run the command line version of mochify.

~~The problem resides in the return code of the operation, it is always 0-ok. I believe it is because what will fail now is the bundle but not the process itself.~~

I figured out the problem in cmd.js. Anyways I'm still failing since ```.bundle(cb)```  doesn't call ```cb``` if the tests fails, only if the bundle fails. I need to catch this case somehow so it is possible to ```process.exit(1)```

Can you please have a look?
Thanks
F
Can you also check one thing that is not covered by the test cases please: When using `--watch`, it should not exit on errors, neither bundle errors nor test errors. Thanks.

I'm very much interested in a PR for making Mochify work with Istanbul! I'd love to have both options, Istanbul and Coverify.