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? :-)
Wrong call of board:available_moves in player

+1 for dialyzer
style: indentation
Implement and export missing gen_server callbacks

Now gen_server is happy about board and player
Dialyzer happy with game module
Add spec in game module
Add spec in player module
Merge pull request #1 from MirkoBonadei/dialyzer

Make dialyzer happy