Various fixes - #177

Here is a list of changes I have made (hopefully I did not forget something):

• Dropped support for PHP 7.
• Fixed converting Any element. Since I actually do not need this element I only went so far to make the convertion work when this element is used. It seems to work fine, even wrote a test, but I'll let you be the judge of that.
• Fixed few "visit" methods that were missing Choice and Group as child elements.
• Fixed converting Choice that is inside of a GroupRef that has maxOccurs set to unbounded converted as property of that type instead being an array.
• Fixed properties/methods duplicated in extending class.
• Properties are now protected by default.
• Return type hint is now "static" as some IDEs complain how methods do not exist in extending classes when it is set to "self".
• Added support for "fixed" and "default" values. For "fixed" values, the setter still exists in PHP code but is not present in JMS definition. This was the simplest solution since I did not wanted to mess with PHP inheritance rules.
• Added "strict_types" option to configuration.
• Added types to properties/arguments/return values in some places.

Note that all my work is based on converting XZuFi 2.3.
So far everything seems to work as expected and soon I will be using generated classes and (de)serialization in customer code, which will in it self be a test if I broke something 😄

Oh and another thing, if you run the tests (all of them) it might happen that OTA tests fail but it seems that it is a bug in JMS as one interface defines "parse(): array" but class implementing it returns a string.