Added to packagist.

Fixed wrong exceptions. Thanks to /u/muglug

Style changes.

First release of Vouchers

• Adds the ability to generate vouchers and voucher codes.
• Adds the concept of bags (collections) to allow vouchers to work in sets.
• Adds the ability to write custom generators for voucher codes.
• Adds the ability to write custom validators for voucher codes.
• Adds the ability to pick vouchers at random from a bag.
• Adds the ability to pick vouchers based on a callback.