Features: WASD movement, background colors, updated Makefile

Hello,

Here's a couple features that I thought were worth implementing. Using `./2048 -b` as well as additional colors will change the background color in the `getColor()` function. In addition, WASD movement is now available.  Interestingly enough, using `ABCD` will also move the board around: this is a peculiarity in `getchar()` which is most likely implementation dependent. I don't know if implementing the three-character solution is possible because WASD only returns one code per character. For more info see
```
https://stackoverflow.com/questions/10463201/getch-and-arrow-codes
https://stackoverflow.com/questions/15306463/getchar-returns-the-same-value-27-for-up-and-down-arrow-keys 
```

In addition, I happen to get an error when compiling on OSX Mavericks: `warning: incompatible implicit declaration of built-in function 'snprintf'` with `gcc 4.6.3