Refactoring of Key Storage API (to fix extensibility problem)

### Summary of Changes:
* Moved all driver interfaces to new sub-package (com.google.k2crypto.storage.driver).
* Renamed StoreDriver/StoreDriverInfo to just Driver/DriverInfo. (Expecting no name collisions.)
* Fixed extensibility issue with driver API by removing attributes from DriverInfo and splitting Driver into ReadableDriver, WritableDriver and WrappingDriver interfaces.
* Updated javadocs with new driver requirements.
* Move K2FileSystemDriver to com.google.k2crypto.storage.driver.impl.
* Updated all storage unit tests.
* Documented all storage API packages.