Webclient: adapt to new assoc def format (#393).

Webclient API:
- Association.prototype.get_role = function(role_type_uri) is new convenience function

BREAKING CHANGES

When working on an assoc def on client-side you must replace
{{{
    assoc_def.uri                   ->  assoc_def.part_type_uri
    assoc_def.assoc_type_uri        ->  assoc_def.type_uri
    assoc_def.whole_role_type_uri   ->  "dm4.core.whole"
    assoc_def.part_role_type_uri    ->  "dm4.core.part"
}}}

See ticket 393.