Explicit spies

Pending https://github.com/katowulf/mockfirebase/issues/26 so don't merge yet. Bower is currently pointing at a git branch and will need to be updated and rebased for an actual released tag. 

MockFirebase will be removing its behavior of looping all keys on each instance and spying on them. Spies are somewhat expensive to set up and execute and doing so was adding test overhead in the 100x range for MockFirebase and in the 3-10x range for dependents' test suites. 

I've gone ahead and made the very small number of required updates to explicitly spy on methods where needed. This brings the execution time for me on the Karma run from ~15s to ~5s.