Converting formatter tests and core to mocha

And this concludes mindless copy-paste exercise... Here are the stats:

Vows coverage:

=============================== Coverage summary ===============================
Statements   : 98.05% ( 1260/1285 )
Branches      : 95.54% ( 750/785 )
Functions      : 98.64% ( 362/367 )
Lines              : 98.05% ( 1258/1283 )

Mocha coverage:
=============================== Coverage summary ===============================
Statements   : 98.12% ( 1302/1327 )
Branches      : 95.75% ( 788/823 )
Functions      : 98.68% ( 373/378 )
Lines             : 98.26% ( 1298/1321 )

Once this is merged in, I will create another pull request to remove all vows tests and replace them with mocha