[cmd:load] Set source of the identities

Using the option --source, the user can set in the registry
where the information comes from.
[tests] Define test base case for load tests
[cmd:load] Add class to load identities based on distinct formats

'IdentitiesLoader' is an abstract class defined to import identities
based on differnt formats. Some existing code from 'Load' was refactored
into 'EclipseIdentitiesLoader' class to import identities using Eclipse
identities JSON format.

New exception 'LoadError' has been also develop to raise errors when
something goes wrong loading identities.
[mg2sh] Fix typo in documentation
[cmd:load] Support for importing identities using grimoire format