Autoloads file for MELPA packaging/compatibility

By adding `sublime-themes-autoloads.el` this directory can now be packaged
automatically by MELPA and can be packaged for Marmalade. While it serves the
same purpose of the ;;;###autoload statements,
`package.el` doesn't currently support parsing files that don't match the
name of the package. Thus, every theme would need to be package individually.
However, I think to reduce the amount of noise (there are a lot of packages
available) it's best to just package all these themes as a single installable
package.