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

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.

Here is a rewritten test suite for Regex DataHandler. This rewrite aims
at being lighter, more readable, accurate.

The previous test was testing FormatString.construct_data which is
irrelevant to this test case. Thus, I rewrote it to test what Concat
does : format a format_string to initialize FormatString with.