rework conversion logic again

Eliminate unnecessary params from coerce(). Make it an abstract
method of XmlSchemaObject so all classes have to implement it.
Make the method return DOMNodes or simple types in an array.
fix the attribute code in XmlSchemaComplexType - was copy/paste error
fix some constructor invocations for types and compositors
fix simple types to return arrays
fix complex type attribute handling

Merge the arrays being returned; and unset handled attributes.
first implementation of XmlSchemaSequence::coerce()

Handles sequences with only 1 type of element.
add a small README