Added posibility to fetch a single application with a known id.
Changed the PodioProvider interface to require a ItemParser.

The ItemParser is now injected in the REST request calls rather
than set as a member field on the PodioProvider implementation.

I also moved the ownership of the ItemParser from the Podio
facade to the different PodioProvider implementations.
Refactored the package structure slightly.

 - Separated the Filters from the Providers into a separate package.
 - Moved ItemParser to the top level package.
 - Renamed ItemParser to PodioParser.
Added get short, mini and micro subsets of an app.
Fixed a regression bug in ItemProvider.
Cleaned up the domain package.
Reverted JSON date parsing to String.

There seems to be a mismatch in how Gson expects the dates to be formatted
and how the API returns them in the JSON response, hence going for the
String notation again.
Aligned the Field type domain objects naming conventions.

Basically renamed ApplicationField to ApplicationReference and CategoryField
to Category. It was considdered a bit superflous to have the "Field" term both
in the class name and the package name.
Added an "undefined" state in the Field Type enum.

There was a need for this in order to conveniently handle conveniently handle
null pointers during testing field type in the client code base.
Added "item create" and "item update" to the SDK.