Support for STIX v1.1 and *experimental* profile validation

* Updated schema directory to STIX v1.1
* Added **experimental** `SchematronValidator` and `ProfileValidator` classes to validators.py
* Modified all validator classes to accept filenames, file-like objects, `etree._Element` or `etree._ElementTree` instances as `validate(...)` arguments. 
* Added example profile and example instance document