support for GAC assemblies in REPL

fixes #308.

Note that with our current REPL structure, when we reference from REPL with #r nothing happens (no code execution) - only the reference list with assemblies will get updated. At that time we don't really want to even check GAC because we would have to use the messy Fusion API.

So the possible error (assembly not found) will happen and be caught on next step, when any real line of code is executed. At that time user will see the error, and the offending assembly will be removed from the reference list.