Call Bullet.enable=true multiple times

The strategy of method aliasing creates stack level too deep errors
because we lost track of the original methods.

I wanted to enable and Bullet only within some of my specs in my test
suite. Then, I did it by setting the attribute accessors
n_plus_one_query_enable, unused_eager_loading_enable,
andcounter_cache_enable. But It should be more clear to user
`Bullet.enable = false` directly.
Adding spec to disable and reenable Bullet
Merge pull request #155 from aq/master

Call Bullet.enable=true multiple times