Optional SLF4J support without adding compile time dependency

Hi,
I know this has been discussed (and rejected) before, but I am hoping that this solution can satisfy everyone.

https://github.com/nuvolabase/orientdb/issues/95

This change adds a an optional runtime dependency on SLF4J. If SLF4J is detected at runtime then it is used, otherwise logging is delegated to java util logging.

In addition if orientdb was to ever be run on android then this mechanism could be used to swap in the android logger...

This code modification doesn't add SLF4J as a dependency by default as the scope is specified as 'provided'.