classes use 'Piwik' name, change to something more specific

[kevinb456]

The classes used for the mod use the name Piwik;

class ControllerModulePiwik extends Controller
class ModelToolPiwik extends Model

etc.

so when the mod is installed or settings are changed, all the operations are done with the 'extension' GET variable of 'piwik'. This could get in the way of an "official" Piwik module if they ever have one?

Change to be something more specific to this mod.

[kevinb456]

Also remember to think about backwards compatibility with previous users, as all the database settings will be stored based on the 'piwik' module name.