style: indentation
Better symbol() type specification

@mirkobonadei

* A symbol could be an atom `x` or an atom `o`
* A board it's a list of symbols or atoms `none`

With this we could export the symbol type and be sure that a move could
not insert the atom `none` or every other atom besides `x` or `o` what
do you think? Pretty cool dyalizer eh? :-)