better method naming

Better method naming. Backward compatible.

2.0.0

1. Added some more ISO codes.
2. It accepts both lower and upper case as input.
3. It returns the same code if not found rather than 'Unknown'.
4. phoneCode() method to return country phone code by ISO code added.
5. continent() method to return continent by ISO code added.

1.1.0

Update composer.json

1.0.0

Create README.md