Not including extras by default. Also added subspecs to selectively include extra components

pod 'QuickDialog' will now only include the 'core' elements

pod 'QuickDialog/Core’ will include the core elements
pod 'QuickDialog/Extras' will include all extras
pod 'QuickDialog/QPicker' will only include the QPicker extras
... etc...
Merge pull request #611 from bofrese/podspecs

Not including extras by default. Also added subspecs to selectively include extra components