[XLRSTATS] syntax cleanup + major fixes

This commit carries the following changes to the XLRstats plugin:

- syntax cleanup
- make use of the new getCmd function defined in b3.functions
- make use of self.getEventID() to retrieve events ids
 - make use of the new event handler system in XLRstatsPlugin
- make use of the new event handler system in CtimePlugin
- make use of the new event handler system in BattlestatsPlugin
- added debug information on plugin configuration file loading
- correctly declare Kfactor attribute in PlayerStats class
- replaced variables names using python built-in keywords
- match also 'yes' and 'no' keywords in !xlrhide command
- added missing call to super constructor in XlrstatshistoryPlugin
- added missing call to super constructor in CtimePlugin
- added missing call to super constructor in BattlestatsPlugin
[TESTS] added tests for !pluginfo command + updated old test cases to comply with the new changes