Notify applications post-install

This PR replaces the `IAppUpdate` framework with a much simpler scheme, that is also compatible with native C/C++ applications. Closes #14.

## Spec

After applying updates, Squirrel will scan the new installation directory for EXEs which match one of these two characteristics:

* The EXE is a .NET assembly and it has an AssemblyMetadata key of "SquirrelAwareVersion" with a value of "1"
* In the English-language block of its PE Version header, it has a key of "SquirrelAwareVersion" with a value of "1"

Every EXE that is found that matches will be invoked with:

* `/squirrel-installed [new-version]` - Invoked on initial install
* `/squirrel-updated [new-version]` - Invoked whenever the EXE is updated
* `/squirrel-uninstall [version]` - Invoked when the application is fully uninstalled