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
added outlined ShowCommand
added RoslynReplEngine

Conflicts:
	src/ScriptCs.Engine.Roslyn/RoslynScriptEngine.cs
updated RoslynModule
ensure proper inheritance in the engines
retrieve vars using reflection instead of static analysis
replaced ShowCommand with ReferencesCommand, UsingsCommand and VarsCommand
added HelpCommand
added HelpCommand meta
fixed null references in the engines, show only the latest value of the variable
added tests for ReferencesCommand, UsingsCommand and VarsCommand
changed LocalVariables from property to method, added RoslynReplEngineTests
added GetLocalVariablesMethod tests to MonoScriptEngineTests
added missing Description to AliasCommand, ensure HelpCommand works correct with the command dictionary (instead of list)