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)
Merge pull request #830 from filipw/feature/show

Added support for :references, :usings, :vars and :help in REPL
prefixed FileSystem locations with scriptcs_
changed command line to assume REPL when no command or script name is passed
- this is to allow running modules without having to specify -repl, e.g. scriptcs -m "migrate"
added migrate module