Safe colors ability via require('colors/safe')

Finally creating a pull request for this issue: https://github.com/Marak/colors.js/issues/62 . Tests are passing and the API for normal (prototype modifying) colors remains the same. For colors that don't modify the String prototype, use require('colors/safe'). Second commit makes build module a dev dependency (so people aren't pulling that in [its a lot of stuff] from npm all the time).