start of ideal 3.3.1 integration
Upgrade to version 3.3.1
Implement ClientInterface for StandardClient

Enable the interface for the client again and adjust the methods
to reflect the required naming and variable types.
Use @inheritdoc as method documentation
Remove _requestIssuers() method
Remove unused methods
Use Zend\Http\Client for http calls
Mark currency and language as read-only properties

According to the iDEAL documentation, the only supported currency
is euro and the default and recommended language is Dutch. Therefore,
to protect users from changing it remove the setters.
Load expiration period and currency from transaction

The transaction will contain default values to ease usage.
Place setCurrency back for status requests
Validate response in send() method
Use only DOMDocument for XML manipulation

Remove all SimpleXML syntax to only support DOMDocument, DOMElement
and DOMNode syntax. This is an in-progress commit to use only DOM
manipulation.

All HEREDOC strings must be converted into DOMNodes as well.
Use Signature object to sign and validate the XML

The sign and validation process is abstracted to ease the testing
of the client.
Remove autoloading files

As of common practice, remove autoloading files and only rely on
autoloading inside the Module class or autoloading via composer.
Write documentation about the client usage
Make standalone less explicit
Update issuer listing example code
Add missing parentheses for method getIssuers()
Add badges
Add section about general usage