Add pluggable persistent storage support
Added support for QSettings based backing store
Implemented a extendable peristent store as per the design here:
https://github.com/mandeepsandhu/o2/wiki/Pluggable-storage-support-in-O2

More backing stores can be implemented by extending the base class
O2AbstractStore and implementing it's 2 methods.
Add pluggable persistent storage support
Updated README.md to show correct usage of new classes
Merge pull request #9 from mandeepsandhu/master

Add pluggable persistent storage support