Optimize code, comply with golint and add the directions api

The directions api was added As I am exposing more of the mapquest api I needed also to refactor the code a bit to optimize it (avoid code duplication etc...) 

I propose we use for the files the naming convention of the mapquest api of their REST api (such as geocoding.go, directions.go, staticmap.go, elevation.go..., see http://open.mapquestapi.com/).

Also I propose to comply with Google's Golint, as strict style guides can make code more readable and collaboration easier. (https://github.com/golang/lint)