Added parallel support for fetching dependent Nuget packages.
TODO: Discuss versions of packages to document (v2, v4, v4.5) and subsequent fetching of dependent packages.

Patch for NormalizeLinks - something to do with NuDoc not handling <see cref="" /> elements correctly. Need to check this out in detail (tested with Gibraltar.Agent package).

Added fix to fetch package by ID as well as by name, and to allow manual entry of package name/ID to download.

Replaced package.GetLibFiles with package.GetFiles("lib"). GetLibFiles only returns DLLS and only downloads them. GetFiles("lib") returns all files in "lib" folder in package, including XML - which NuDoc needs to be able to parse.