Deleted duplicate file

Deleted duplicate RaindropViewEngine.cs file from the
/Raindrop/Raindrop/Raindrop folder.
Moved backend into sub-namespace

The backend was moved into a sub-namespace instead of private classes to
enable testing without cluttering up the API.
Changed expected template extension

Somehow the expected extension for template files got changed to .html.
This changes it back.
Changed expected template source to TextReader

The backend now expects a TextReader and name string instead of a path
string. This should make it easier to substitute arbitrary data sources.
Also, the data source is now opened with the correct sharing attributes
(FileMode.Open, FileAccess.Read, FileShare.Read).