schemaVersion type should not be "decimal"

[jschaeff]

The XSD states:

		<xs:attribute name="schemaVersion" type="xs:decimal" use="required">
			<xs:annotation>
				<xs:documentation>The schema version compatible with the document.
				</xs:documentation>
			</xs:annotation>
		</xs:attribute>

StationXML/fdsn-station.xsd

Line 88 in fc7feb2

<xs:attribute name="schemaVersion" type="xs:decimal" use="required">

It should be another type allowing to make a 3 numbered version.

Discussion for schema versionning is in #113