[2.0] Remove app.boot

[bajtos]

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:

// 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.

[ritch]

LGTM