Fixe integration tests when run as a complete fixture.

Previously the test database was created / destroyed on fixture setup /
teardown. This meant that tests were not isolated from each other and some
of the tests would fail depending on the order they were run in. This
commit creates and destroys the database for each test, isolating them.
Update to .NET 4.5
Convert core LoveSeat methods to async / await semantics.

Return async in all methods where we trigger an http request.

Async methods should always return a Task to ensure proper exception
handling and method composition with the TPL.
Use JSON.NET from NuGet
Add ability to pass enumerator into bulk document collection.
Add support for constructing Documents from anonymous objects.
Replace usages of ExpectGet with SetupGet.

Moq has deprecated ExpectGet.
Update ViewOptions to use Stringbuilder
Use ConfigureAwait(false) for internal async / await calls.

This is really ugly, but considered good practice in an async API where we
don't know who our consumers are (e.g. WPF / ASP.NET). This shouldn't make
any difference to ASP.NET usage but, for UI consumers, this will prevent
deadlocks and unnecesary context mashalling.
Close http responses in CouchBase.cs. Fixes #31
Cope with null response stream in GetResponseString.
Update readme to reflect async / await changes