Restructure

I converted everything over to the style that Google used for [their Github API client](http://godoc.org/github.com/google/go-github/github). Check out my version on [godoc.org](http://godoc.org/github.com/inhies/gonexmo).

I feel like this is a very logical way to do things and will make future expansion seamless. We'll be able to add on Client.Voice, Client.Number, Client.Search, etc. 

My apologies for taking so long with this and the giant messy commit on January 7th. Everything got mixed up and I had to step away from coding for a week or two which didn't help. I'll go sit in the git corner-of-shame...