Adds start of the Windows Phone 8 crypto provider.
Implements the hash function using BouncyCastle.
Finishes implementation of the WinPhone8 crypto provider.
Switched from NuGet distro of WinRTBouncyCastle to my own build that is strong-name signed.

This is the release build from exactly this version:
https://github.com/AArnott/w8bouncycastle/commit/447e2336fbeb12174ed5f27f2df08ef6afa7cf07
Adds WinPhone8 unit test project.

SymmetricEncryptionRoundtrip test currently fails.
Fixes failing WinPhone8 encryption round-tripping unit test.
Simplifies GetCipher method.
Symmetric encryption is now fully specified by the ICryptoProvider.
Adds Windows Phone 8 implementation of ICryptoProvider.