[Build] Move some of the system dependencies into nugets

[Therzok]

We currently require the mono libraries and gtk# packages installed in the GAC.

http://www.monodevelop.com/developers/building-monodevelop/#windows

These are also dependencies on mac, but they're provided with mono. We should nugetize these components to allow for fewer system dependencies when building on Windows, allowing us to integrate via VSTS.

VS bug #597612

[Therzok]

Dependencies we have:

• ICSharpCode.SharpZipLib - need to unify this with monodoc nuget
• monodoc - StrongName monodoc.dll api-doc-tools#229 [NuGet] Ship monodoc.dll in build directory api-doc-tools#228 monodoc.dll.config does not substitute monodoc_refdir api-doc-tools#227
• Mono.Posix - easy, should use Mono.Posix.NETStandard
• Gtk#

[mhutch]

PR to create monodoc package at mono/api-doc-tools#246