Fixed quotation escape in GMLWriter

All strings containing quotation marks written into GML should be escaped.
Added JUnit tests for GMLReader and GMLWriter dealing with quotations

+ Test in GMLReaderTest: reads an escaped quotation property from
example.gml
+ example.gml: Escaped quotation property in Vertex 3
+ Test in GMLWriterTest: writes a quotation mark correctly to GML
% GMLWriter: One escaping added, otherwise, it didn't work (see
GMLWriterTest)
Fixed regression with example.gml

% As I changed example.gml earlier, the tests must be adapted
Merge pull request #470 from guerda/master

Fixed quotation escape in GMLWriter