From DAL to new pyDAL

Implementation strategy:
* git submodule in gluon/contrib/pydal
* module import to sys.modules in gluon/contrib/__init__.py

Major changes in web2py:
* dal imports rewrite
* DAL subclassing to add web2py's features
* pyDAL exceptions handling
* removed old DAL tests
* introduced specific web2py DAL tests (to be expanded)

TODO:
* check applications for old imports
* check contrib modules for old imports