Collapse `attr()` and `Attr()` into a single export

[wadetandy]

Currently we export attr and Attr in our module. The former is to support the ES5 model API:

JSORMBase.extend({
  attrs: {
    name: attr(String)
  }
})

while the latter is a decorator for typescript (and hopefully soon standard JS) class syntax:

@Model()
class MyModel extends JSORMBase {
  @Attr name : string
}

With a bit of runtime checking, we should be able to collapse these into a single export to help avoid confusion going forward.

[wadetandy]

cc @wycats @richmolj