Only load ssl default ssl certificates once (#35)

Fixes I/O in the event loop

Each update was creating a new ssl context which was
doing blocking i/o to load the default ssl certs

sense_energy/asyncsenseable.py

@@ -2,6 +2,7 @@
 import aiohttp
 import json
 import websockets
+import ssl
 
 from .sense_api import *
 from .sense_exceptions import *
@@ -14,6 +15,8 @@ async def authenticate(self, username, password):
             "password": password
         }
 
+        self.ssl_context = ssl.create_default_context()
+
         # Get auth token
         try:
             async with aiohttp.ClientSession() as session:
@@ -45,7 +48,7 @@ async def async_realtime_stream(self, callback=None, single=False):
         """ Reads realtime data from websocket"""
         url = WS_URL % (self.sense_monitor_id, self.sense_access_token)
         # hello, features, [updates,] data
-        async with websockets.connect(url) as ws:
+        async with websockets.connect(url, ssl=self.ssl_context) as ws:
             while True:
                 try:
                     message = await asyncio.wait_for(