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
Is it still the case though? The page says that it is the case for SafeHandle in structs, by we don't have any structs in the native code (only passed by args).
> In one special case (SafeHandles in sturctures that are passed as parameters in P/Invoke functions) we are not calling AddRef/ReleaseRef because we would have to keep track of the original handle, but this data is currently not available in the release/cleanup code for marshalling.

This blog post talks a bit about this issue as well (at the end) http://tirania.org/blog/archive/2006/Dec-16-1.html

If you could do the test to see if SafeHandles passed by value as argument, or obtained via a return value are working fine on current version Mono, it would simplify the pinvoke code a lot by removing most of the #ifdef needed.