implemented find project by ID

Added Project#find function which, given a project_id, returns that project.

Included test for the new function.

Dependent on this feature was the ability to specify that the `ClientResource#get` a single item as opposed to a collection. I wasn't able to figure out how to get it to detect it, so I added a `:single` flag to the `#get` function. Given this flag, it creates a single object (eg: `LetsFreckle::Project` when calling `LetsFreckle::Project#find`. If you know of a better, way, I'm down with that as this method feels a little heavy.