Update to new API, unify CLI, add functions for backup, autobackup and PSK handling

[rtpt-timoboettcher]

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This pull request adds support for the APIv2 and various functions, complete with examples

• change shebang to python3
• update library for current unifi API
• adjust parameters of other example scripts to provide the same command line interface (--debug, --no-ssl-verify, --certificate)
• example script to list access points
• library support to create and download backups, with example script
• library support to download autobackups, with example script
• library support to read and change PSK of a network by name or UUID, with example script

[dsully]

Would love to see this merged.