Add method to make complete url for server

Method `url` looks more useful then `url` property.
Make `url_for` works as expected when live server is running

Explicitly set application ``SERVER_NAME`` for test suite and restore original
value on test teardown.
Add docstring for `live_server` fixture
Add note about `live_server` to pypi page
Use 5000 as default port for live server

For better compatibility with Flask-Testing.
Update version number