Handle exception in invoking

User can change exception handling strategy via Config.
By default, error is raised only in developement environment.
Load rails gem in top of nyanko

It is not only used from railtie but also from other files.
v0.0.2
Alias ext to unit, ext_action to unit_action

This is for backward compatibility of Chanko API.
Chanko had called unit as `extension` in the past,
but changed the name to `unit` for public release.
v0.0.3