add ability to specify library directories to resolve references to third party libraries

relates to fsharp/FAKE#309

API docs can't be generated due to unresolved reference to third party library
please try to generate APIDocs for FSharp.MedadataFormat, it has a reference to Razor, and will fail at *Parsing assembly* step.

With this feature you can specify directories where to search for referenced libraries and APIDoc generation works just fine.