Added support for express 4.x

Core code comes from https://github.com/mozilla/connect-cachify/pull/42. I took his PR and manually merged the relevant parts. Additional tests added to ensure all of the expresses are *really* being tested.

Original commits:
* 587518018b73c7acc39f411b918635081485d240
* 62a5bd69a9c8ae1a413da2616ab0b7894e2ddbf2
* 5a9b544b7266009dda3edcf0d6cab4ef0bb770a5
* 8ae7c94a48b95838bef130af10cf4337556a665c
* 9c219d1e19cce5944c55816e6b61333c1c4250ca

Thank you very much @kmchugh!