Detect whether ember-cli was installed via npm or via npm link.

A mechanism to detect whether ember-cli was installed via `npm install` or via `npm link`.

Idea for using this information:
- If installed via `npm install`: Use local ember-cli package if the user is in a ember-cli project
- If installed via `npm link`: Always use global ember-cli package. (Because local version would be outdated)

This is useful for https://github.com/stefanpenner/ember-cli/issues/231