Test: added local Mocha testing with grunt-mocha

Todo by our resident build masters:

1. Find more elegant way of specifying test files.  Currently hardcoded array.
1. Possibly merge common tasks of `test-mocha` and `saucelabs` :

```coffeescript
"clean:dist",
"assets",
"js",
"css",
"copy:tests",
"assemble:tests",
```