support for multiple boxes and username authentication

[CWempe]

This PR is supposed to fix the following issues:

• FritzOS 6.50 FritzOS 6.50 #1
  (was already fixed in this branch)
• How to add two fritzboxes? How to add two fritzboxes? #3
• How to separate the graphy to a new node entry How to separate the graphy to a new node entry #4
  At least I had that issue. 😕
• support authentication with username support authentication with username #9
  I am not sure if this breaks support for authentication without username!

Tested with FRITZ!OS 6.51 and 6.80

[Tafkas]

Set default username to None:
def get_sid(server, username=None, password, port=80):

[Tafkas]

If username is not None, use it here, otherwise use the old url

[Tafkas]

Hi Christoph,

thank you for your PR. I think it looks good. One minor thing, though. You mentioned the authentication with username/password might break the password-only option. How about this:

check if there is a a username set with os.getenv instead of os.environ. The latter will raise an exception in Python 2.7, while the former will return None.

I added some comments in the code as well.

[CWempe]

Thank you for your suggestions.
I think this should handle non-username scenarios now.

btw: The API request worked with the username-URL even when authentication was configured for password-only.

As someone who never used python before, I am pretty proud of myself. 😄

[kripton]

Any reason that this has not been merged? I'd also like to monitor multiple boxes instead of just one