Support for MasterCard "2" series BINs

Added support for MasterCard "2" series BINs

Remove expiration date to string

Remove expiration date to string, since users may need any number of formats.

Minor tweak to toString() for expiration date.

Minor tweak to toString() for expiration date.

Enhancements

Clean-up of code, javadocs, and minor bug fixes and usability enhancements.

Java 6 Support

Java 6 support for older Android systems.