Introduced the 'Store' interface to the SDK.

This would be the interface to the local storage implementation.
Added the first implementation of the local store.
Added the `LocalStoreRequest` + minor `Store` API change.
Refactored the `Provider` responsibility chain slightly.

Previously the `VollyProvider` was responsible to hold
and provide the an instance of the `Client` implementation.
This responsibility has now been moved a bit down in the
hierarchy, to the abstract `Provider` implementation. In
this way, custom providers, that don't necessarily provide
content, doesn't have to extend the implementation
specific `VolleyProvider` in order to get hold of the
`Client`.
Enabled access to local store through the Podio facade.
Isolated the `ClientProvider` in it's own stand-alone class.
Enabled means of closing a store.