Create `extend()` API so any Express module instance can be extended

Previously, only the the Express module instance which is a
`peerDependency` of this package would be extended with this package's
functionality. This now exports the `extend()` function which allows
any Express module instance to be extended.
Add `augment()` export which adds `expose()` to an existing Express app
Add jsHint option to ignore strict `this` in the `expose()` function
Add tests for `extend()` and `augment()`
Tweak tests where methods are `describe()`-ed
Add HISTORY entries for `extend()` and `augment()`
Merge pull request #5 from yahoo/extend

Create `extend()` API so any Express module instance can be extended