add python setup.py install guide
fix README code block
remove dependency_links
install pytest-pythonpath by pypi not github
fix https://github.com/jollychang/robotframework-appiumlibrary/issues/29 and https://github.com/jollychang/robotframework-appiumlibrary/issues/27
via https://github.com/bigsassy/pytest-pythonpath/pull/2#issuecomment-67441228
version 1.2.1