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 seeing anything, and this  Pull Request now says "nto Doxense:master  from unknown repository" which is probably not a good thing :) I fear that we have broken GitHub. Maybe rebase the PR will fix it. If not just start a new PR from scratch after pulling from master ?