package for nuget with strong names

Uses a `SIGNED` compiler flag to conditionally compile the
`InternalsVisibleToAttribute` needed for testing, and some other flags
in msbuild to sign the assembly when compiling it for nuget.

This probably needs more work, I predict trouble when trying to compile
a library with local dependencies like `Acceleration.Hmac`, which wants
`Acceleration.Collections`. I guess if everything gets installed via
nuget that might work, albeit a little silly.

Also splits the `Acceleration.MappedReader` tests into their own
assembly. Initially done attempting to get the strong names happy, but
ended up not doing that.
strong name for Acceleration.Versioning