Fix possible collidation of userExtSource logins

- only for extSource type 'IDP'
- now every login in userExtSource type of IDP is unique
- add some notNull testing to method addUserExtSource in entry
- add new method for geting all userExtSources by extLogin and extType
- add simple test
Merge pull request #359 from stavamichal/fixPossibleCollidationOfLoginsInUserExtSourcesTypeIDP

Fix possible collidation of userExtSource logins