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
Too late I already merged the commits in This has been merged in 835f75cf622677e96fee95298c1ca2ede8df6fd2.

Can you do the new changes starting from this commit, and update this pull request? (or if it breaks GitHub, maybe create a new one)