Fixed the GetItems key argument to be a KeyValuePair
Added StateField. Changes how item fields are inititialized, so it's no longer required to set exernal id/add to coll on item update/create
Implemented Filter Items method
Added Task Service
Implement basic CRUD operations
Merge pull request #9 from shearnie/master

Added Task Service
Update README.md
Merge commit '9ad046cc49eba73f48ddb6e520ffb94e3c91f48f' into dev

Conflicts:
	Podio.API/Services/ItemService.cs
	Podio.API/Utils/Item.cs