How to configure Mocha and Chai added in README

Added optional instructions how to install and configure Karma to use Mocha and Chai.
Changed Mocha & Chai to replace instead of add

Jasmine and Mocha don't like to be used together. You'll have to choose between them. Updated the README with optional replace instructions instead of adding.
Merge pull request #31 from bgever/patch-1

How to configure Mocha and Chai added in README