Stix2

* Fixes bug with incorrect TAXII parameters (169).
* Fixes bug with constructing patterns (171).

* Adds the Workbench layer API.
* Adds checks to ensure valid type names are provided.
* Supports parsing generic custom STIX 2 content without needing to create classes for them.
* Fixes "Not JSON serializable" error in TAXIICollectionStore.
* Fixes bug with parsing JSON files in FileSystemStore.
* Fixes bug with Filters in TAXIICollectionStore.
* Fixes minor bugs in the patterning API.
* Fixes bug with ListProperty containing DictionaryProperty.
* Fixes bug with parsing observables.
* Fixes bug involving optional properties with default values.
* Changes custom observable extensions to require properties to be defined as a list of tuples rather than a dictionary.
* Changes Filters to allow passing a dictionary as a filter value.
* Changes `get_dict` to a private function.
* `taxii2-client` is now an optional dependency.

* Fixes issue with PyPI.

* Adds functions to dereference relationships.
* Adds a function to get an object's type from its ID.
* Reorganizes DataStore api, renaming: stix2.sources to stix2.datastore.
* Fixes various bugs involving CustomMarking, creating new versions of objects
with custom properties, and a missing IntrusionSet property.
* Drops Python 3.3 support.

* Adds `creator_of` function to easily get the Identity that created an object,
and a `serialize` function for fast serialization without sorting properties.
* Fixes bugs with DataStores and with adding custom STIX content to Bundles.
* Supports filtering on any property, not just common properties.
* Includes internal changes to make it easier to support multiple versions of
the STIX specification.

* Adds data stores, object factory, and the environment layer.
* Supports pattern expressions and data markings.
* Fixes some bugs with timestamp precision and parsing of bundles.
* Includes support for custom object types, custom properties, custom observable
objects, and custom observable extensions.