Contentful

Added
* Added `timeout_s` to Client. [46](https://github.com/contentful/contentful.py/pull/46)

Fixed
* Entries that had already been processed when coercing RichText fields are now properly omitted. [41](https://github.com/contentful/contentful.py/issues/41)

Fixed
* Errors are now being correctly propagated through the chain of relationships.
* Links in `RichText` fields, that are published but unreachable, due to not having enough include depth on the request, are now returned as `Link` objects.

Changed
* Updated `requests` version due to a vulnerability found in versions `2.19` and below.
* Included resources for embedded entries and assets in Rich Text fields are now properly serialized to `data.target` instead of the top level `data`.

Changed
* Removed Pandoc depedency from installation

As `RichText` moves from `alpha` to `beta`, we're treating this as a feature release.

Changed
* Renamed `StructuredText` to `RichText`.

Added
* Added support for `StructuredText` inline Entry include resolution.