require Global.app to be set before creating a GlobalID
add :app option
Adding tests for restricting lookups by class
Adding option to locators to limit classes allowed
ensure GlobalID.app is set back
test situations when app is nil or GlobalID is nil
use fetch to get the option with a default
make the error message more clear
Simplifying logic to check for :only restriction
Moving find(:only) checks into GlobalID instance
Merge pull request #12 from tony612/require-an-app-to-be-set-before-creating-a-globalid

require Global.app to be set before creating a GlobalID
Increasing test coverage for find restrictions
Clarifying #locate documentation for :only
More tests for failure cases of find(:only)
Removing accidental stray newline from GlobalID
Merge pull request #14 from code-lever/class-restriction

Limit lookups to specific classes via :only
Apps can hook in their own locator.
Apply Jeremys suggestions.
Attempt to fix remaining issues.