Improve the next-step function, implement prev-step
Disable next/prev buttons when not usable
Show crude current rule information
Move import-board back to cludoku.board

Now the interface accepts a string, instead of a filename. The
Clojure implementation slurps a file before calling import-board,
and the ClojureScript implementation reads the file using the
FileReader API and passes the contents to import-board.
Use the FileReader API to load arbitrary sudokus
Integrate the board name in the status info and reset-game
Update README