Updated README & fixed reporter exit code

The first change is an update to README.md to match current Bamboo Node.js Plugin tasks & new mocha-bamboo-reporter feature (the MOCHA_FILE env var).

Second change concerns the mocha-bamboo-reporter. So far executing Mocha would exit with an exit code greater than 0 if there were test failures. This is not an expected behavior, since we can have quarantined tests in Bamboo. In such case we should let Bamboo decide if this job execution is a failure or success AFTER parsing test results & applying quarantine rules. But if the exit code of Mocha was not 0, Bamboo would treat the job execution as failure anyways - that's why the error code returned by mocha should always be 0.