Add beamer support

Exporting to LaTeX using `org-beamer` does not work. This is because `beamer` as a backend is not provided for. This could easily be solved by changing lines like `(equal backend 'latex)` to `(or (equal backend 'beamer) (equal backend 'latex))`; it accounts for both `LaTeX` and `beamer`. Any chance you could make these changes? Your blocks are too great not to be used in `org-beamer`!