Add double checked locking to Memoizable::Memory#fetch

* This is based on the comments in
  https://github.com/headius/thread_safe/issues/15#issuecomment-25480218
  on how to handle this exact issue with memoization.
Merge pull request #9 from dkubb/double-checked-locking

Add double checked locking to Memoizable::Memory#fetch