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 this page up to date? http://www.mono-project.com/SafeHandles seems to date from 2005-ish
It seems that they support SafeHandle as arguments, and also as return values. Or are there some known bugs that make them unusable in practice?

If we duplicate all the pinvoke code, it will be difficult to maintain if they add new methods in fdb 3.x, because currently I don't have any way to test the code in Mono.