Added min/max metadata to numerics and a default binning strategy

This commit adds new min/max metadata to all numeric columns. That
metadata is used in the automatic binning of breakout fields. This
commit also extends MBQL to indicate that a breakout column (or
columns) should be binned using a "default" strategy. The query must
also include the maximum number of bins. Using the number of bins with
the min/max values, the bin width is computed and used in the SQL
query to group data.