Fix ldap convertSID2Str() / Remove BCMath dependency

Follows https://github.com/owncloud/core/issues/10991#issuecomment-56026802

The existing implementation uses the bcmath extension in order to mitigate problems with integers on 32-bit systems. Ironically, the existing implementation does not seem to convert sub-ids with a leading bit set correctly on 32-bit systems (due to `unpack('V')`). This PR addresses that issue and furthermore removes the bcmath dependency (due to largest integers (48-bit) fitting float).

The tests in question passed on one of my 32-bit systems (Raspberry Pi).

@DeepDiver1975 @blizzz 
Throw exception instead?