Async all the way.

AbstractService<T> had the following issues:

- the type variable T was reused in multiple places.
- all calls had to specify the type, but only one call to AddAsync<T>
actually uses a different type. Therefore, only that method accepts a
type parameter.
- nested callbacks for tasks are not necessary in .NET 4.5.

As a "side effect", all method calls in the services are now async, and
output `Task<T>` instead of `<T>`.  They also changed names so as to
more easily know that something broke.
Paymill does not need to be a static class.

Instead, introduce a constructor that takes the required parameters.

To get the individual service, use properties on the Paymill object. The
old method did not in any way guarantee that the service you got was an
AbstractService<> of any kind.
Giant ReSharper pass.

All files are changed. This fixes all issues reported by ReSharper with
the exception of unused methods in SandboxConsole\Program.cs.

It also completely removes the HttpClientRest class, as this class no
longer did anything after ReSharper had done its thing.
Add JSON.net package and enable package restore.
Rename service methods and read only results.

Makes calling methods on services more natural:
paymill.Clients.GetAsync() vs paymill.Clients.GetClientsAsync().

Also, the return value is now a IReadOnlyCollection<T>, so the intent is
more readily apparent.

This also seems to lay the groundwork for moving more of the
functionality in AbstractService.
Move more functionality into AbstractService.

The individual service implementations now do much less work, following
the DRY principle.

Four interfaces, corresponding to the individual CRUD operations have
been created. Then the combinations of those interfaces that we use
(e.g. full CRUD, only create and read) are exposed on the Paymill class.

The services are no longer public, so externally you can only work with
the interfaces.
Reworks filtering into statically typed queries.

New syntax is:

await paymill.Transactions.Query.Take(1).Skip(3).GetAsync();

For now this is missing the Order() operator.
Support ordering for queries.
Add unit tests for parsing of incoming data.

Uses actual data from API documentation and proper API calls.

Also restructures internal namespaces a bit.
Get the next set of results from a result set.
Errors are important, but we still want the response.
Rename to PaymillSharp
Merge pull request #1 from digitalcreations/dev-async

Version 0.3