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

Removed the "from .datasource_plugin import DatasourcePlugin" in
Plugin.register

The exact same conditionnal statement throwing error if ptype is unknown
in Plugin.register() is already executed by
MetaPlugType.type_from_name(), which is called right after it. Guessing
which class should be concerned by such a check, I ended up removing it
in Plugin.

Deleting code from old standalone server

Seems to works !! :)

We delete all results with the same UID...

+ bugfixes in show_object_detailled

get_referenced method were implemented for SingleReference and MultipleReference class
They are able to return LeObject instances refereced by them

Hook names were wrong

forget to name the argument

Now when we try to update an object that do not exists a 404 is answered and not a 500 anymore

Deleted old methods that are unused (one of them was triggerring an error will doing nothing)

Now we are able to limit the number of spawned child processes

Now listening address, listening port and max_children count can be configured using the Settings module

Adding a cache for app module reducing seriously routing overhead

Now all "empty" values are deleted from the resulting array

There were manual import through importlib, we have to replace them with LodelContext calls.
Actually the plugins module is in a dirty state....

Replace some old pure python import by LodelContext calls
Change the plugins imports methods to use LodelContext instead
Fixes dyncode exposure method