Change pip classifier to Beta
Rename internal methods, add docstrings to public methods
Fix bug in Connection.sock and Connection.is_alive()

If the connection is closed, calling is_alive() would raise an exception rather
than returning False. The bug is now fixed and is_alive() returns the correct
value as expected.
Bump version
Improve reliability of Connection methods

* Improve reliability of Connection.is_alive(): now returns True/False instead
  of raising an exception if connection is already closed.
* Improve reliability of Connection.close(): now returns immediately instead of
  raising an exception if connection is already closed.
Add automatic heartbeat capability