Modified .gitignore

Added folde doc/_build to .gitignore
How to create controls

Added 'How To' about control creation
Comments in Control.js

Added comments to Control.js
Modified PermalinkButton

Some changes into PermalinkButton control. Integrated method like trigger
in OLControl
Added FullScreen control

FullScreen control creates a new window using permalink control with map in fullscreen mode. Created a fullscreen new parameter in map.options.yaml to manage this functionality