added AliasCommand for REPL command aliasing
added REPL command test file and changed the initialization in REPL
added behavior test
addressed code review feedback
Merge pull request #769 from filipw/feature/alias-repl

Allow REPL command aliasing
red: added RoslynScriptEngineTests.ShouldAllowUseOfScriptHostExtensionMethods to demonstrate that extension methods on script host do not work