create setup.py and distribution directory structure

[lczub]

To distribution TestLink-API-Python-client as an Python module, a setup.py file is needed. It is also a common practice a) to split source and test into separate directories. b) split the code into several files

Suggestion for a directory structure

    setup.py
    /src
        /testlink
                 __init__.py
                 version.py
                 TestLinkErrors.py
                 TestLinkAPIClient.py
                 TestLink.py
    /test
        test.py
    /examples
        TestLinkExample.py