Configure mocha using Grunt rather than mocha.opts

[gaurav21r]

In line with our strategy, grunt provides a very productive, declarative way of giving configuration options to tools, not to mention standardized. Its much better to give options to mocha too using grunt than an opts file.

Further, this can also mean we can have different options for different grunt tasks (say one for the CI server one for the local environment etc.) which is not possible with opts.

@riteshe63 Please let me knoe if there is something we can write in the opts which we can't in the Gruntfile?

This will require some changes in #52