Enable assembly signing (#10)

[22222]

Enables assembly signing in the CsvTextFieldParser project.

One question is whether the private key should be kept a secret or if it should be checked in. I decided to check in the keyfile based on Microsoft's advice in Strong-Named Assemblies:

If you are an open-source developer and you want the identity benefits of a
strong-named assembly, consider checking in the private key associated with
an assembly into your source control system.

Which also says:

Do not rely on strong names for security. They provide a unique identity only.

More documentation about strong naming:

• Strong naming
• dotnet corefx Strong Name Signin