fritzbox_helper: TLS, username authentication and custom form fields

[mtdcr]

First, I'd like to thank you for fritzbox-munin!

I don't use munin, but I found your repository looking for a library implementing basic session ID handling for fritzboxes.

Summary of changes:

• Add optional TLS connection
• Add authentication method using username and password
• Make use of params parameter of requests.get()
• Remove fake User-Agent

If you'd like to try a TLS connection from a Debian-based system, you might want to set the following environment variable: REQUESTS_CA_BUNDLE=/etc/ssl/certs/ca-certificates.crt

[mtdcr]

In the meantime, I've reimplemented the functionality I needed in a slightly different way: https://github.com/mtdcr/fritzxhr Feel free to take my PR, but I'm closing it for now, because there was no activity for more than three weeks and I'm not going to pursue it further.