Simplify bundle task

Returning a stream from a gulp task is one of the ways to indicate async completion/failure
https://github.com/gulpjs/gulp/blob/master/docs/API.md#async-task-support
Fix src issues

* js extension in require is implicit
* lodash needs to be a dependency
Replace _.extend with xtend

Browserify has to bundle all of lodash which is quite heavy
Unlike Node you have to be somewhat concious of API size and minimizing bundled code
Final src: set module.exports to an empty objected extended with a and b
Fix tests

* Use karma-as-promised instead of a gulp plugin
* Inline karma config in gulpfile
* Delete unused dependencies
* Properly configure test paths
Generate test coverage

100% - yay!
Set main script to the correct location
Merge pull request #2 from bendrucker/master

Fix all the things