List paging & Linting

ongoing PR, do not merge.

Adding `Next()` methods to all list resources an satisfying `golint` for every package at the same time.


- [x] accounts
- [x] act
- [x] applications
- [x] common
- [x] conference
- [x] notifications
- [x] numbers
- [x] recording
- [x] sms
- [x] transcription
- [x] twiml
- [x] usage
- [x] voice

closes #28


Analogously to the `List` structs, it makes sense to have a `sms.Resource`. It works fine as long as the user is not instantiating the object.

However, anytime the user is creating the object (to pass it to a POST method), they should be canonical names, like `Message` for sending a SMS