Implement evil option flag for dynamic exec

Makes the ability to dynamically execute code within the VM an opt in behavior. If not explicitly enabled then attempts to do so will throw an appropriate error.
Implement pending.wrap helper method
Make nextTick cancelable using pending.wrap
Implement safeExec helper method
Move loadInContext exec to pending events
Add test for failure for script exec cases
Halt execution of the script loader on error

This also ties the script loader into the pending event architecture so we can be certain that all scripts execute in the event of calling emit early in the page lifecycle.
Properly handle errors in onEmit callbacks
Fix merge issue
Update per review
Merge pull request #25 from walmartlabs/failure-fixes

Handle client code failures cleanly