Added more convenience methods to the utility class.

Now one can conveniently get the current time as seconds and format
Java date objects to date-only strings.
Introduced the v2 request interface.
Introduced the v2 filter object.
Introduced the v2 client interface.

The Request implementation is now responsible for delegating the
HTTP request to the Volley library (previously it was the job of
the client implementation).

The Client implementation is however still responsible for
authentication requests, which is reflected through the client
interface now (previously the responsibility was exposed through
a special-case request object).

The client has also been reduced in complexity as the request
queue and worker thread spawning has been removed from it. The
Volley library provides for both features which is relied upon
now. This removes an entire layer of thread and enables a much
leaner implementation which is much easier to debug.
Updated the provider infrastructure.

Incorporated the Filter objects into their corresponding provider
classes in order to reduce the package foot print. The providers
will now also use the v2 request and filter infrastructure.

NOTE: This commit will break the old session management
infrastructure. The new client implementation is replacing it,
though.
Updated the client infrastructure.

The Podio Android SDK now uses the v2 client implementation. The
old implementation is removed as of this commit.
Cleaned up package structure.

The Volley specific client, request and provider implementations
have been isolated in their own package. This also required some
minor accessability changes in some other classes.
Renamed 'PodioRequest' to 'Request'.

This in order to better comply with a generic naming convention,
as does the other classes do within this API package.
Modified visibility of provider filters.

This in order to enable heritage of providers and their filters.
Fixed id-as-integer error (should be long).
Minor code hygiene changes.
Enabled the view provider in the Podio facade.
Added support for force refreshing tokens.

This was done as a temporary evil. The functionality should not be
there at all, but in order to be able to deliver this refactoring feature
without having to include a major rewrite of our Android client as
well, this was seen as an acceptable trade off (for now).
Refactored the way auth errors are handled.