The UniqID datahandler has to call the datasource
	LeObject has to be able to determine wich parent class define its UID datahandler
	[NOT IMPLEMENTED] the datasource should be able to select with an abstract LeObject as target ( see #92 )

Avoid useless impossible import by putting confpsec in a dedicated file

- bugfixes in connection string generator
- bugfixes in garbage collection of opened connections

This is used for the tests that needs to have access to a connection to be able to drop the test databases

- now we have a LodelHook to trigger migration handlers loading
- datasources initialization in dyncode is now trigger by a hook (lodel2_plugin_loaded)
- datasources loading by dyncode is better

- The LeUpdateQuery implementation is broken or at least have huge performance issue
- Moved LeObject exceptions and LeQuery exceptions in lodel.leapi.exceptions

- now datasources are declared with two conf keys : read_only and identifier
- the old datasources plugin hook is now declared in lodel/plugin/core_hooks.py. This file is imported after lodel/plugin/hooks.py in the loader.py

- Enables connections saving & auto cleaning
- Preparing possibility to declare a datasource as read only

- Now delete & update methods use filters & rel_filters