(#13) - use binary search for options.start

FWIW, this also passes the PouchDB test suite.

The only thing I'm concerned about is the `exclusiveStart` option.  It seems like it should be XORed with `key === options.start` rather than just ORed, but the test suites pass anyway.  Also confused as to why the localstorage-down tests pass even though there's no mention of `exclusiveStart` in that code whatsoever.