Add device discovery (#25)

* add (untested) discover mode

* Keep discovery and normal communication separate, uppercase magic consts

This sepearates the earlier test code for discovering devices,
and adds 5 sec timeout for gathering responses from potential devices.

This commit also uppercases magic constants.

Discovery & communication tested with HS110.

* update readme with example how to discover devices, pep8ify

examples/discover.py

@@ -0,0 +1,9 @@
+import logging
+from pprint import pprint as pp
+
+from pyHS100 import TPLinkSmartHomeProtocol
+logging.basicConfig(level=logging.DEBUG)
+
+for dev in TPLinkSmartHomeProtocol.discover():
+    print("Found device!")
+    pp(dev)