Use HTTP POST for `query` and `queryGraph`

(Re: #35)
* The `httpMethod` for `_httpRequest` in `query` and `queryGraph` is
POST instead of GET to allow for longer queries which would otherwise
time out.
* `query`, `baseURI`, `limit`, `offset` parameters are passed as POST
`msgBody`. For browser, `msgBody` uses the window.FormData class,
following the `createDb` method.
* Additional test suites created for longer queries and for testing
whether offset & baseURI parameters work through POST.