Added installer.nsi script to generate setup.exe.

Installer requires administrative rights to run.
Installer does the following:

- Installs Microsoft .NET 4.0 redist if
  not installed on the system. This is the main bulk
  of the installer, as the redist itself weights about
  50 Megabytes.
- Installs Microsoft XNA 4.0 redist if not installed.
- Installs the game to specified directory.
- Installs uninstaller and registers that uninstaller
  with Windows's "Programs & Functions" in Control Panel.