[2.0] Remove `app.boot`

Modify `app.boot` to throw an exception, pointing users to the new
module `loopback-boot`.

/to @ritch please review

/cc @crandmck this affects the example the documentation and examples:

```js
// current docs
app.boot(__dirname);

// the new way
var boot = require('loopback-boot');
boot(app, __dirname);
```

Note that `loopback-boot` can be used with loopback 1.x apps too, thus in theory the changes can be made in both 1.x and 2.x versions of  loopback docs.

Depends on: strongloop/loopback-boot#15
Related:  #348.