support installation via PyPI

TestLink-API-Python-client should be installable from PyPI

[PYTHON27]\Scripts\virtualenv [PYENV]\testlink
[PYENV]\testlink\Scripts\activate
pip install TestLink-API-Python-client

conditions are described in PyPI Tutorial

[x] extend setup.py (categories, long description, package name)
[x] extract Installation, Usage and Testing sections out of Readme.md into separate new doc/*.rst files
[x] divide unit test directory into two directories, one for tests with TestLink interaction, other for standalone tests
[x] transform readme.md into readme.rst
[x] create changes.rst files

lczub / TestLink-API-Python-client