(#961) - api.changes: return promise

Return promise with a `cancel` method when calling the changes api
* while it uses the `taskqueue`
* with `{ since: 'latest' }`
(#961) - Follow up: fix jshint errors
(#956) - Server Replication

Introduce `options.server` which allows to initiate server replications
via `PouchDB.replicate`.
(#915) - Check Database exists before deleting in node.