Contentful

Added
* Added support for `StructuredText` field type.

Fixed
* Ensure all arrays sent as parameters to the API are properly serialized.

Added
* Added support to reuse entries. This is a performance improvement, which is disabled by default due to backwards compatibility. All users are highly encouraged to enable it and test it in their applications. Inspired by rezonant in [contentful/contentful.rb164](https://github.com/contentful/contentful.rb/pull/164).

Added
* Added support of environments to `sync`.

Added
* Added support of `environments` functionality.

Added
* Added filtering of invalid entries from API responses.