Changed to .NET 3.5 project

Rfc2898DeriveBytes has been part of .NET since v2.0 and therefore this
library can be made compatible with lower .NET framework runtimes.
Unfortunately the library also makes use of LINQ which requires .NET
3.5.

This commit contains the following changes:

- The target framework for the SimpleCrypto project has been changed to
.NET Framework v3.5 (client profile).

- Unused references have been removed from the SimpleCrypto project.

- Unused using directives have been removed from the ICryptoService and
PBKDF2 classes

- The using statement around Rfc2898DeriveBytes has been removed because
System.Security.Cryptography.DeriveBytes doesn't implement IDisposable
in Assembly mscorlib.dll, v2.0.50727