Feature/62 api adjustements

Changes:

• remove getUsernameAvailability and getMailboxAvailability
• mailbox -> emailAdress in parameters and GET paths (no interface change though)
• key (where the key to a password reset or account creation token data was meant) is renamed to "token"
• remove special casing of password handling between LDAP and Basic (both require the plain text password now)
• throw RuntimeExceptions setting the cause properly
• use ' instead of " in log messages
• improve developer and API documentation
• remove unused methods
• safely skip display name operations when not privileged to do so