DataTypes and new UserService and WorkspaceService method

Here's a new release with a few improvements, cleaned up code and breaking changes (the *Item classes have been moved to another namespace).

There is still a lot of work left... And possibly some breaking changes :/

Changes

• Data types (like 'Workspaces', 'Lists', 'Elements' and so on) now extend AbstractDataType which makes life a little easier :)
• EntryCollection->entries is now called EntryCollection->listEntries as in the raw API response

New methods

API

• Convenience method getUserService

Users

• getCurrentUser

Workspaces

• getUsersForWorkspaces