Support fetching all projects for an API key
Introduce ProjectsResource

The Client no longer has behavior specific to
a resource/endpoint. A new ProjectsResource
class is introduced and is now a user of the
Client.

The ProjectsResource supports the following
operations:
 - all (list all projects, can be scoped by user
   by passing in an optional user_id argument)
 - find (get a single project by id)
 - export (export a project by id + a hash of
   options to use for generating the request XML)
Support exporting a project as an image/pdf/…
Add support for creating a new project
Add spec for ProjectsResource#find
Add the drawing model
Update spec name
Add a simple spec for the ProjectsResource#all method
Add spec for the ProjectsResource#create method