Add mocha tests for all generator scenarios

* Added mocha tests to describe various scenarios provided with the generator:
    *  meanjs generator without sample app
    *  meanjs generator with sample app
    *  express sub-generator tests
    *  angular sub-generator tests
*  Refactor the option which allows user to choose the module to check if module/ directory already exists since tests fail as the tests are run individually
*  Added travis badge to indicate the build status. This will require simple setup and configuring on travis-ci.org