Add support for planned and estimated counts

[lmcmicu]

Issue #13 implemented exact counts. This issue is to add support for planned counts and estimated counts.

Note that this will require an extra argument to the fetch_as_json() function to specify which kind of count is requested (the default should be exact).

It's possible that there is no analogous optimization that we can use for SQLite. In this case we can possibly output a warning if the requested count type is not exact, but otherwise it should be transparent to the user.