Merge branch 'feature/versioned-api'
Add Go 1.2 to the supported versions
Add some comments to explain the Client properties
Set an user agent to properly identify the client

Borrowed from go-github
Update the Domain struct with the current fields

I had to temporary disable the Renew because it was relying on a field that is no longer part of the domain model.
Temporary workaround for Renew

RenewWhoisPrivacy is not part of a Domain struct, but for now let's keep it here, since it's used for the Renew method.
Sort the Record fields according to the official serializer
Early return if the request creation failed

Do not attempt to set any header.
Document NewRequest
Use a namespace-based approach for API calls

I like the approach adopted by go-github where you use

    dnsimple.Domans.Create

instead of

    dnsimple.CreateDomain
Add ability to get a single record
Add some documentation
Fix mismatching test messages
Rename Domains.SetAutorenew to Domains.SetAutoRenewal
Add ability to create a domain
Document the creation of a new domain
Creating a record requires to pass the newRecord
Be a little bit more restrictive in path generation
Export NewRequest outside the package
Move payload serializer inside the NewRequest

There are still a few inconsistencies in our API. The convention "if payload is here, serialize it" is more appropriate.