Update the build script in prep for strong naming of assembly CSHARP-143

- Change Version property to PackageVersion
- Derive the AssemblyVersion and AssemblyFileVersion from the PackageVersion value using the rules discussed in the JIRA ticket to minimize pain of binding redirects
- Removed the PACKAGE_VERSION environment variable since we're now just specifying it in the build project
Merge branch 'mapping' of https://github.com/LukeTillman/csharp-driver