Add the snk "private" signing key to the public repo

This follows the guidance from Microsoft in
https://github.com/dotnet/corefx/blob/master/Documentation/project-docs/strong-name-signing.md

> Firstly, we would recommend that these open source projects check-in their private key
> (remember, strong-names are used for identity, and not for security).

Changes to project configurations can follow (but aren't urgent):

- Remove ReleaseSigned etc
- Sign all builds (including debug)
- Get rid of conditional keys in AssemblyInfo for InternalsVisibleTo

This fixes issue nodatime#443.

Author: jskeet

.gitignore

@@ -22,9 +22,6 @@ StyleCop.Cache
 # Auto-created thumbnails
 Thumbs.db
 
-# Noda Time release (private) key
-NodaTime Release.snk
-
 # Build output directories (anywhere)
 **/bin/
 **/obj/