[WO-612] Implemented requireTLS option to explicitly require STARTTLS support

This addresses #28 

I rearranged the tests so there's a large diff. Relevant tests can be found in lines [L90-L211](https://github.com/whiteout-io/browserbox/blob/76baf18ce919322d1d7bb5a7c23a91074ca5908c/test/integration/browserbox-test.js#L90-L211)