Added ScriptEnvironment on ScriptHost

Added `ScriptEnvironment` property `Env` on `ScriptHost`. Fixes #279
It includes:
 - [x] References
 - [x] Namespaces
 - [x] RootScriptInfo (`FileInfo` for the executed script) (Fixes #225)
 - [x] Script (the currently running script)
 - [x] ScriptArgs (Fixes #278)
 - [x] Assembly (Fixes #244)
 - [x] LastException