Support paying by token

* add support for guzzle/psr 2.0
* add support for billerCode (required for using stored tokens)
* add support for cancelUrl (via return bar)
* add support for hide flags
* update to standard getCreateToken/setCreateToken
* fix scrutinizer coverage upload
* fix some docblocks with incorrect param types

Merge POST & GET data for CompletePurchase

Make sure works on PHP 5.6/7.0, fix alias and properly define webhook…

… data

Add webhook parsing support

Update to Omnipay 3

Fix another minor Travis typo

Initial commit