Multi versions

The following enhancements are included in this pull request:
* Can perform schema/best practice checks against multiple versions of STIX. Closes #20 
* Check for titles on major constructs. Closes #5 
* Broke out validators.py into validators package with separate modules for Schematron, XML Schema, and STIX-specific (Profiles, XML schemas, Best Practices) validation classes.
* Added stubs for future best practice checks
* Fixes to `sdv.py` argument parser
* Removed `--schema-dir` option from `sdv.py`. Schema directories are set via `settings.py`