v2.0.0

This pull request is for v2.0.0 of the stix-validator.

* Installable API via `setup.py` (and `pip` once this is merged)
* Bundled STIX v1.0 - v1.1.1 schemas
* Added unit tests (needs many more)
* Documentation (API/scripts/installation/getting started) for Read The Docs (once merged)
* Changed versioning scheme to [semantic versioning](http://www.semver.org)