Update src/marionette.layout.js

regions may also be defined as a function, if you want to wait to define them until runtime.

initializeRegions invocation in constructor has been moved after ItemView initialization.
changed regions handling in initialize function
Update src/marionette.layout.js
setting up failing specs for layout regions defined as a function
merging in the ability to specify layout regions as a function
release notes and docs for layout specifying regions as a function
build