small go style changes & go get error with ./build import

go-ified slices/func args & fixed import error with go get tool from ./build import, also automatically go'fmted the code (sorry).  

Hi Aaron,

Nice game, love it.. 

I can undo all the gofmt formatting and re-request if you'd like.  Here's all I really changed:

- only have to put iota once and list the rest of the const values (will work the same)
- func functionName(x int, y int, z int)  can just be: func functionName(x, y, z int), did this throughout
- slice[0:n] can just be slice[:n]
- fixed error for anyone using the go get tool, your import "./board" is better written as "github.com/AaronO/go2048/board"

Let me know if you want me to undo the gofmt, it's built right into my editor, i didn't intentionally mean to change 99% of the codebase. 

Also, awesome program, thanks for the opportunity to read your code.