Added spiral-switch-to-last-clojure-buffer

[dotemacs]

As discussed in https://github.com/Unrepl/spiral/issues/11

Allow the users to switch back from from the REPL to the previous buffer belonging to the project.

There is one issue, which I only caught when I tried to use the keybinding: intentionally the keybinding for spiral-switch-to-last-clojure-buffer is the same as for spiral-switch-to-repl-buffer. The same as in CIDER.

The only problem is that spiral-switch-to-repl-buffer key binding shadows spiral-switch-to-last-clojure-buffer. Let me fix this. But until then please let me know what you think of this function. Thanks

[volrath]

@dotemacs looks good so far.