External dependencies control

[OlegZee]

[OlegZee]

This is very early idea. The point is that system should take care on external dependencies. In the most trivial form it should just provide paket, nuget package managers.

However I'd like it to allow defining external libraries as usual artifact similar to local file. Build system should load and cache external deps and update the links respectively. E.g. for Csc task: refs ["System"; nuget "Nunit.Framework" 2.6.3]