Associated to multisite loading & context stuff. I choose to commit this
change in master because it is important to not break the fact that
the buildconf HAS TO be importable safely outside any context.

No knowing what datahandler ought to be used for em_groups, I simply set
'text' in place of it. To be changed accordingly

The Plugin class and discover steps were updated. Some test are deleted
because they were obsolete (discover method do not take any argument
anymore)

Warning, may contains some bugs !!!

For the moment there is only uninstalling by name that works because #207

Clean take care of deleting plugins with same name and different versions
(the lower versions will be deleted)

Now in verbose mode we are printing all plugins, even those with duplicated
names and different versions

the action is named plugins

For the moment a new action for lodel_admin has been added : list-plugins
With two options : verbosity and output formatted in CSV

fixes #205

Now discover is run when we want to know the list of installed plugins.
+ bugfix in module name generation (only in the constructor) to take care
of possible subdirs