Began work on adding unit tests for unicode support. Refactored bindings.quote_attrib and bindings.quote_xml in the process. Lots to do still. [ci skip]
More unicode tests, though it still needs a lot more testing. Modified bindings to correctly call quote_xml and quote_attrib
Modified quote_attrib and quote_xml binding methods to handle
encoded strings.
* Assumes that the string has been encoded as
  bindings.ExternalEncoding.
* Returns a string encoded as ExternalEncoding
* Added unit tests to check this behavior.
Modified encoding_tests to call common _test_equal method when testing title, description, and short_description values.
Added docstrings to to_xml() method. Fixes #230
Added test for code example in #220. Modified count_affected property to cast input values to int.
Attempted to reduce unwanted implicit encode/decode calls
Added comments to quote_xml and quote_attrib
Merge pull request #233 from STIXProject/unicode

Unicode