refactored roslyn out of core to fix #90 in preperation for #80
refactored roslyn out of core to fix #90 in preperation for #80
refactored code to bring ScriptPackSession back into the core

1. We refactor ScriptPackSession so it is just a data bag with some
methods.
2. That allows it to be easily passed to the engine which figures out
how to apply it.
3. We move the initialize and terminate into ScriptCs.core.
added [InheritedExport] back onto IScriptEngine
moved ScriptHostFactory into Roslyn since the script host is Roslyn specific
Merge branch 'issue-90' of github.com:nberardi/scriptcs into regbuilder

Conflicts:
	src/ScriptCs.Contracts/IScriptPack.cs
	src/ScriptCs.Core/DebugScriptExecutor.cs
	src/ScriptCs.Core/ICompiledDllDebugger.cs
	src/ScriptCs.Core/IScriptEngine.cs
	src/ScriptCs.Core/IScriptHostFactory.cs
	src/ScriptCs.Core/ScriptExecutor.cs
	src/ScriptCs.Core/Wrappers/ScriptEngineWrapper.cs
	src/ScriptCs/Program.cs