Refactor project into better module hierarchy.

See David Symonds's post from
https://groups.google.com/forum/#!topic/google-appengine-go/dNhqV6PBqVc

> I gave the rough sketch of the recommended layout in my last
> paragraph, and it sounds like David Byttow will be able to give a real
> example soon, but here's a sketch of a layout:
>
> $GOPATH/src/github.com/foo/bar/app/app.yaml
> $GOPATH/src/github.com/foo/bar/app/app.go
> $GOPATH/src/github.com/foo/bar/moduleA/moduleA.yaml
> $GOPATH/src/github.com/foo/bar/moduleA/moduleA.go
> $GOPATH/src/github.com/foo/bar/pkg/quux.go
>
> app.go and moduleA should import "github.com/foo/bar/pkg". Don't try
> to import the stuff in app.go or moduleA.go from anywhere.
Use templates.

* Create view directory with logic to handle templates.
* Make appError a model in the model directory.
* Rename mailpin.go to app.go
Refactor AppHandler into model directory.
Put incoming email logic into its own file.
Improve page viewing.

* Render page views within a template.
* Separate page view logic into controller.
Store page body as a string.