Unifi-python-api

fixed functions assign_existing_admin() and invite_admin() to support controller versions 5.9 and higher because of observed changes in controller behavior

added function/method assign_existing_admin()
fixed code for function/method invite_admin()
minor code tweaks for improved readability

made regex to extract cookies case-insensitive, though this would only be needed for corner cases this is still RFC compliant (https://www.w3.org/Protocols/rfc2616/rfc2616-sec4.html#sec4.2), thanks to ericfri
changed the layout of the README file to put more emphasis on the installation options and the basic example, and less on the list of methods/functions

updated both READMEs
added cmd_stat() function, can currently only be used to reset DPI counters for the current site
added optional group_id parameter to list_firewallgroups() which allows you to select a single firewall group, thanks to VWT-Dan for the suggestion

added methods stat_ips_events(), set_ips_settings_base(), supported with controllers 5.9.10 and higher
thanks go to leonardogyn for providing access to a 5.9.29 controller and test network for test/validation purposes

general code cleanup:
- added spaces throughout Client.php following PSR-2 guidelines that were skipped in the previous commit
- added visibility to constructor and destructor methods following PSR-2 recommendations
- declare user and password properties and visibility following PSR-2 recommendations
- updated README to add recently added functions/methods
- minor general changes to the README