Create documentation install option for SB

[vlidholt]

Make it possible to install the Cocos2D docset documentation in Xcode directly from SpriteBuilder.

• Add a Install API Documentation in Xcode menu item in the Help menu.
• When selected ask the user permission to modify the Xcode docs directory, then install the docset
• The docset needs to be generated during the build process, zipped up and put in the generated folder
• When first run, SpriteBuilder should ask the user if it wants to install the documentation
• SpriteBuilder should detect if it's been updated, and offer to install updated documentation on first run after update