Some improvments

- Adding support for storing double (as a string) in the shared preferences.
- Remove static keyword since it need context.
- Add remove key method.
- Add clear all method.
- Add put & get float.
- Add put & get long.
- Change Boolean to boolean (primitive).
- add getAll()
- Add register & unregister listener for changes.

*I've deleted the repo, so see the changes in the Files changed tab*