Now the context manager handles lodelsites paths

Thoses modification has two goals :
- make lodelcontext fit with the new FS organisation implemented by the
lodelsites datasource
- make the dyncode exposure more "flexible"

Note : no solution implemented for MONOSITE

Conflicts:
	lodel/plugins/lodelsite_datasource/fs_utils.py

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.

At this point the loader in lodel/plugins/multisite/run.py is theorically
complete & correct. But some features are missing in order to be able
to test & debug it ( refs #275 ).

Conflicts:
	lodel/plugins/lodelsite_datasource/fs_utils.py

Now we are able to insert a lodelsite

no debug has been made on those piece of code

As a primary_key field has to be unique, added a check at DataHandler
instanciation to make sure a field set as a pk is not set as non-unique

Modified the shortname's DH attribute value from uniq to primary_key

Switched the shortname field's DH from Varchar to Regex

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 !!!