Handled a new parameter called "scope".

If scope is not given than the legacy account search is implemented
otherwise if scope is LDAP then the search is only in LDAP user base or
DB for local seafile db user base.
The returned JSON list now has also a "source" attribute about user
origin.
Corrected list initialization and return response for not a valid scope
value
Merge pull request #258 from bits4beats/accounts_list_scope

Handled a new parameter called "scope".