Add support for BSON MinKey and MaxKey.
Add support for NumberLong.

Fixes #142

Note that, due to restrictions in the Ruby driver < 2.0, Genghis is unable to determine whether an integer was explicitly stored as 32 or 64 bits, it can only tell whether that number is bigger than 32 bit integers can hold.

This currently does not set ini_set('mongo.long_as_object', 1) in genghis.php, but it probably will, since that makes things actually work as one would expect most of the time.

This also doesn’t handle converting 64-bit integers received from the client as integers. If genghis.php is running on a non-64-bit backend, decoding a JSON request from the client with 64-bit integers will probably do dumb things.