editing purge and purgeAll to return thenable

[eclifford]

I ran into the situation where I needed to know when the underlying calls to removeItem were finished executing (since they are async). My PR simply propagates the thenable resolution once all removeItem's resolve. With this update calls to purge and purgeAll are thenable.

[rt2zz]

This is definitely desired functionality.

Concerns:

1. should also support callbacks
2. I dont think the purgeAll implementation works as it relies on getAllKeys returning a promise

[rt2zz]

@eclifford thanks for the work! I adapted it and applied it to the coming v4 PR.

Regarding my comments, I have decided callbacks is a feature that can be added later, and I updated the default getAllKeys implementation to support promises.