Use a build properties file instead of custom build actions to copy files

This change is based on the following two observations:

1. When content items use the `CopyToOutputDirectory` property instead of build actions to copy contents, the copied files are automatically cleaned up by the build system during clean/rebuild commands. This is especially useful when the names of the copied files change (e.g. when the NuGet package is updated), because old versions of the library are automatically removed the next time a project is built.
2. When using the **\*.props** feature of NuGet, there is no need to update the custom build action (if any) which the user has specified, reducing the likelihood that the user introduces some sort of error when committing the updates to source control.

I tested the functionality of the **\*.props** mechanism locally by manually updating the contents of the **.nupkg** file and installing it into projects locally. **This means the following very important items have not been tested yet.**

1. The **UpdateLibgit2ToSha.ps1** script *should* update the properties file, but I have not tested it.
2. The **LibGit2Sharp.nuspec** file *should* be updated to properly incorporate the new feature (and remove the now unnecessary installation scripts), but I have not tested that it produces the correct NuGet package during the standard build process.