Added ScriptPack<T> base class
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
Revised filesystem test name and refactored into new class

fixes test in #296
Followed XUnit convention

Of putting tests in a class with the name of the method they test.
Merge pull request #311 from SpikeTheMaster/dev

Revised filesystem test name and refactored into new class
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
Merge pull request #287 from khellang/script-pack-base

Added ScriptPack<T> base class