fix package resolving (using Nuget.Core)

fixes #10 and #15

the new way to resolve Nuget packages, runs off Nuget.Core.

The workflow is:
1) If working directory has no packages folder / or packages folder has no packages.config in it - load nothing
2) Load package references from packages.config from <current>/packages/ or <absolute_path>/packages (the latter if executed file is referenced via absolute path)
3) For each package reference try to load the nuget package (if not found, add to error list) - this fixes #15
4) Find all DLLs inside the nuget package that match the framework version specified in packages.config (if none found, add to error list) - this fixes #10
5) If there are errors/mismatches => throw Ex and return list of missing packages/dlls
6) If no errors, return list of DLL paths for further processing