Improve support for some XSD features

[martinv13]

A few fixes to solve issues discussed in #15:

• short_name is actually required in some cases despite being an optional argument -> set a default value
• support of recursive XSD definition -> propose a minimal support (e.g. not correct but does not fail) by dropping the fields that make the schema recursive
• support of xsd:list types added
• support of complexTypes with an attribute with the same name as an element of the same complexType added
• support of sequences with unbounded max_occurs containing elements also with unbounded max_occurs