Fix: Fixes no-unused-vars to check /*globals*/ (Fixes #955)

Fixes #955. New property on escope variable changed to `eslintExplicitGlobal` as per recommendation on #964. That's the only new change; see #964 for other line notes.

This changes the default back to the correct `"all"` for the **no-unused-vars** rule; it had been set to `"locals"` at some point.

It also augments the rule (which according to @nzakas should have been there all along, thus this is a Fix) to check for unused explicitly declared globals via `--global` or `/* global */`. This probably never worked before, since the plumbing wasn't really in place.