Add color for --help when output a tty

IPFS's `--help` output now gets Chrismas lights when the output is on the terminal! 

See [#340](https://github.com/jbenet/go-ipfs/issues/340) 

Color rendering is only when stdoutput is on the terminal.

This uses 
* `github.com/mattn/go-isatty` for checking if the output is on a terminal
* `https://github.com/mitchellh/colorstring` for color coding strings

I was not able to add these packages to Godep. It probably should be `make vendor` or `godep save -r ./...` but these don't do the trick. What is up with that?


Here is what it looks like for the standard Ubuntu color scheme:
![ipfs-colors](https://cloud.githubusercontent.com/assets/816895/5157208/84115404-72c8-11e4-92e5-8fd949d0633e.png)

keep common variables grouped (as they were):

```Go
```