Changed interfaces of R.NET.

* REngine.SetDllDirectory is obsoleted. User should set "PATH" environment variable instead.
* REngine now requires Initialize method before evaluating R scripts.
* RDotNet.Environment renamed to RDotNet.REnvironment to avoid confliction with System.Environment.
* REngine.EagerEvaluate method renamed to Evaluate (old Evaluate method to Defer).
* UnmanagedDll (renamed from LateBoundUnmanagedDll) moved to an isolated project.