Move functionality from Config to Pusher object

[leggetter]

@jpatel531 @zimbatm - would love to be able to release this library sooner rather than later! A pragmatic approach to getting this shipped would be very much appreciated.

Main changes:

• Pusher constructor now takes app_id, key and secret. config is an optional named param.
• Config.authenticate_subscription -> Pusher.authenticate
• Config.validate_webhook -> Pusher.validate_webhook
• Config.from_url -> Pusher.from_url
• Config.from_env -> Pusher.from_env

One failing test!

One interesting point to note. In the 1.0.0 branch all tests pass. In simple-paramters the test_initialize_from_env test fails (on my machine) since I'm using Python 2.7.9 and there's a test that sets SSL to true and sync.py checks at least Python 3.4 is installed. I cannot understand why this test doesn't fail on the 1.0.0 branch - it should! I did do some digging and it appears sys.version_info is empty in the 1.0.0 branch when running the test but full of values in the simple-parameters branch. I haz no clue!

[jpatel531]

This looks great to me! +1

[zimbatm]

Put a couple more things in #45 (and fixed the failing test). Now it's much closer to all the other libraries.

[leggetter]

See #45