Complex feature parsing

I am resurrecting Adam  Brown's unpushed work in https://github.com/Adam-Brown/geotools/tree/gml_client_lib. I think at that time he was waiting for Gabriel's wfs-ng branch to be pushed before he could proceed, but I can see the dependencies are all in master now. 

Proposal: http://docs.codehaus.org/display/GEOTOOLS/ComplexFeature+Parsing+and+Building+Support

Going through someone else's code is always a pain. There are some changes I don't quite understand, here are some relevant threads (very old):
http://osgeo-org.1560.x6.nabble.com/Migrate-ComplexFeature-Parsing-work-off-wfs-ng-td4991249.html
http://osgeo-org.1560.x6.nabble.com/Parsing-and-Building-Creating-a-complex-feature-type-out-of-a-XML-schema-td4998747.html
http://osgeo-org.1560.x6.nabble.com/Request-approval-for-changes-to-gt-wfs-org-geotools-data-wfs-WFSDataStoreFactory-java-tc4557246.html#a4557603

gt-main:
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I got rid of some unnecessary changes, including "ComplexContentFeatureSource" and
"WFSContentComplexFeatureCollection"  based on Andrea's feedback: https://www.mail-archive.com/geotools-devel@lists.sourceforge.net/msg29109.html

gt-app-schema:
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I've had a look at the app-schema changes; they are minor and they're fine. 

wfs-ng:
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There are a lot of test failures on wfs-ng, but they are existing failures. There is only 1 new test class in wfs-ng relating to this, I think: XmlComplexFeatureParserTest, and it passes.