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
I'm not sure why you need to create a different set of native calls specific for linux ? If you simplify #ifdef the value of FDB_C_DLL to libfdb_c.so on Linux (like you did above), shouldn't the rest of the marshalling code be the same ? (except the LoadLibrary stuff)