Allow maxreaders parameter to be set

As I requested in issue #6: These changes add bindings to the `mdb_set_maxreaders` function in LMDB's C API, to allow setting the number of available slots in LMDB's reader table. This enables a higher number of iterators to be concurrently active.

The tests aren't as elegant as I'd like, but they do their job and document their hacks.

I added added appropriate documentation to the `README`, roughly following the existing style.

Feedback appreciated.