Added ScriptEnvironment on ScriptHost

Added `ScriptEnvironment` property `Env` on `ScriptHost`. Fixes #279
It includes
 - References
 - Namespaces
 - RootScriptInfo (`FileInfo` for the executed script) (Fixes #225)
 - Script (the currently running script)
 - ScriptArgs (Fixes #278)

It's still missing a reference to the current `Assembly` (#244). I'm not sure how to solve that, especially with the REPL.