Unicode

The following changes were made:
* Copied the `bindings.quote_attrib()` and `bindings.quote_xml()` methods from python-stix.
* Replaced explicit calls to `str(self.valueOf_).encode(...)` and `unicode(self.valueOf_).encode(...)` with calls to `quote_xml(self.valueOf_)`
* Added unit tests for new `quote_attrib()` and `quote_xml()` methods.
* Fixed issue where `FixedOffsetTZ(...)` was being called instead of `_FixedOffset...` (leading underscore missing)

@gtback, I hope this is what you were asking for :)