Update to Swagger 1.2. Fixes #11, #8, #7.

Changes in this PR:

* Moved `public` (swagger UI) out of this module and required `swagger-ui` as a module in `package.json`.
* Added `swaggerDistRoot` option so users of the API Explorer can override any part of the UI (styles, JS, HTML).
* Compatibility with Swagger 1.2 ([spec](https://github.com/wordnik/swagger-spec/blob/master/versions/1.2.md)) ([transition guide](https://github.com/wordnik/swagger-spec/wiki/1.2-transition))
* Model schemas are now correctly populated.
* An `accessToken` can be provided via the header input.

When this is merged, it should be safe to remove `strong-remoting/ext/swagger`, the associated test, and `app.docs()`.

Fixes #7, #8, #11.
Since the explorer has `express: 4.x` as a direct dependency, this can be changed to `require('express')`. Both `require('loopback')` and the function `requireLoopbackDependency` can be removed from the code, as they are no longer used.