Add Travis-CI and CodeCov.io integrations.

[nlutsenko]

This PR adds the following:

• Travis-CI tests on every pull request/master branch
  E.g. https://travis-ci.org/nlutsenko/parse-server/builds/106656352
• Travis-CI tests run in a matrix with Node 4.1 and 4.2
• It's using the latest stable version of MongoDB 3.0 (3.0.8), but we can extend and or change that extremely easy - by simply adding another item into env array.
• Added Codecov.io integration via npm module - this will run only if tests succeeded.
  E.g. https://codecov.io/github/nlutsenko/parse-server?branch=nlutsenko.travis
• Added mongodb-runner npm module, which starts a mongo on pretest/posttest, so we can ensure mongo is always there locally whenever tests are run.

Few benchmarks:

• Average test run is ~1.5min on Travis-CI
  https://travis-ci.org/nlutsenko/parse-server/jobs/106656353
• Average full integration is ~3min on Travis-CI
  https://travis-ci.org/nlutsenko/parse-server/builds/106656352

The only flow change here is that to run tests - we enforce running mongo via mongodb-runner, which I still feel is absolutely fine and the best thing we could do, since it's somewhat a best practice.
Please keep in mind - that this runner is never used in a non-dev environment, but is only installed and used when developing per devDependencies section of package.json.

This superseeds and closes #30.

[nlutsenko]

This is currently waiting on tests to stabilize before merging, and I'll enable Travis-CI integration as soon as we are ready to merge this.