added support for #load and #r in REPL

in REPL you can now do:

> #r "my.dll"
> #load "my.csx"

and these files will be loaded into the session context (csx will also, obviously, execute). 

Since the context is the same, you can then use csx variables/classes in the REPL or you can setup variables/classes in REPL and reference them in the CSX too. #r assemblies need to be in the bin because of Roslyn bug.

To facilitate #r, I maintain a list of assembly references alongside the session state.