Introduce sublime-themes.el instead of autoloads file

@milkypostman's contribution of the autoloads file is only part of the story for MELPA: we also need to grab the package description from somewhere, and that place should usually be a .el file corresponding to the package name.

This commit, then, introduces a `sublime-themes.el` file with a description and author info, and it contains an autoloaded block which
will be used to *generate* sublime-theme-autoloads.el at package installation time.