Enumeration duplicates with special characters like "+"

[jens1205]

I am having the following enumeration

	<xs:simpleType name="UebertragungsverfahrenType">
		<xs:restriction base="xs:string">
			<xs:enumeration value="ADSL2AM"/>
			<xs:enumeration value="ADSL2+AM"/>
		</xs:restriction>
	</xs:simpleType>

Currently normalize just throws array all illegal characters in an identifier, leading to a duplicate const value and a compilation error.

This PR introduces a specialCharacterMapping, which adds a Plus for the + in the name

[jens1205]

I just found this links to #113 and there is another pending PR #114 for this topic