Regression test pouchdb.d.ts against existing pouch-tests.ts

[AGBrown]

I don't know how much of this will be possible, but I would have thought the new pouchdb.d.ts in this repo should be able to pass many of the tests in the old DefinitelyTyped pouch-tests.ts, and doing this may help highlight gaps/structural issues in our implementation.

Probably blocked by #8

[AGBrown]

Actually in a quick and dirty pass at this (no commits until #8 is fixed) this worked with very few changes so that looks good already.