Issue #330 first implementation of TaxGroup
Added PHPDoc class section
Merge pull request #346 from elcodi/feature/small-typo-cartwrapper

Added PHPDoc class section
Merge pull request #331 from elcodi/feature/tax-groups

Issue #330 first implementation of TaxGroup
depend on TokenInterface instead of UsernamePasswordToken
check for empty users in TokenInterface

As `TokenInterface::getUser` may return `null` (specially `AnonymousToken::getUser` will do that)
extract `getCustomerFromToken`
unit-test User\CustomerWrapper