add assembly reference to REPL reference list regardless whether it exists or not. Then catch on exception and remove reference if needed
added tests for verifying that assembly gets removed from REPL reference list if it wasnt found. Also check - if file exists reference by absolute path, otherwise try get it from GAC
change fileloadexception to filenotfoundexception
fixed edge case of null reference
added test for #r in GAC, moved ReplTests.cs to ScriptCs.Core.Tests.csproj to reflect proper structure
Merge pull request #309 from filipw/308

support for GAC assemblies in REPL