Implemented more functions for Application and Item services.
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
Changed Item Model to use ItemReference for Refs property.
Bug fix:  Item.Tags is now a list of strings.
Added support for raw file download.  Added limit header information to PodioResponse.
Implemented more methods for ItemService.
added class file to project
bug fix:  Check ItemField values before enumerating.
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
Merge branch 'dev'