Fixing Issue #1

[Fabs]

There is two errors on Issue #1.

• Sometimes after an error, the stub-collections fail to unregister certain stubs with sinon, causing the test runner to fail forever, since it is not able to stub new collections, nor clean old ones. I am not really sure if this fixes on every case, but I think the lifecycle of the sandbox ends up been cleaned when the tests run again, while the sinon.stub stays there for some reason.
• The private.stubPair() tries to .bind things that for some reason are not a function, that was the _.bind error I had. So I only bind on functions now.

Again, I am not 100% sure of this correction, but so far it is working for me. I will be adding more tests to my meteor project and use this package more.

[hwillson]

Looks great to me @Fabs - thanks for the PR!