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
Why the need for a different implementation for SafeLibraryHandle? Also, it should at least derive from SafeHandleZeroOrMinusOneIsInvalid in order to be safe with marshalling.