Restore state of spied on objects after example

[Nick Giancola]

Block format to auto-cleanup:

Spy.clean do
  spy = Spy.on(...)
end

Manual cleanup:

spy = Spy.on(...)
spy.clean

# or globally:
spy_1 = Spy.on(...)
spy_2 = Spy.on(...)
Spy.clean

Each spy should:

• register themselves globally with Spy (consider threading...)
• track their dirty state and expose a #dirty? method
• implement a #clean method

Spy should:

• implement .clean to clean all dirty registered spies
• implement .contained to automatically clean up after block execution