Added support for Mono/Linux

This patch allows to use the FoundationDB client on Mono/Linux, tested on ver. 3.7 and CentOS 6.5

Just define MONO and it will compile for Mono/Linux
Well that's a bummer, because that means that the Mono branch is vulnerable to resource leaks if anything happen (like a ThreadAbortException) while the IntPtr is still not safely stored in the SafeHandle...

I hope that this will not cause any weird issues when running on Mono.