Add ability to run nuget on non windows systems using mono

Run `NuGet.exe` using mono if system is not `Win32`.

Please check http://monomvc.wordpress.com/2012/03/06/nuget-on-mono/ to find how to run nuget under mono.

Checked on 

    $ uname -a
    Linux hazzik-vpc-deb-2 3.10-2-amd64 #1 SMP Debian 3.10.7-1 (2013-08-17) x86_64 GNU/Linux
    $ node -v
    v0.10.17
    $ npm -v
    1.3.8
    $ mono --version
    Mono JIT compiler version 3.0.6 (Debian 3.0.6+dfsg2-4)
    Copyright (C) 2002-2012 Novell, Inc, Xamarin Inc and Contributors. www.mono-project.com
        TLS:           __thread
        SIGSEGV:       altstack
        Notifications: epoll
        Architecture:  amd64
        Disabled:      none
        Misc:          softdebug 
        LLVM:          supported, not enabled.
        GC:            Included Boehm (with typed GC and Parallel Mark)