StatsComponent should support dates (and other non-numeric fields)

Currently, the StatsComponent only supports single-value numeric fields:

http://wiki.apache.org/solr/StatsComponent

trying to use it with a date field I get an exception like: java.lang.NumberFormatException: For input string: "2009-01-27T20:04:04Z"

trying to use it with a string I get an error 400 "Stats are valid for single valued numeric values."

For constructing date facets it would be very useful to be able to get the minimum and maximum date from a DateField within a set of documents. In general, it could be useful to get the minimum and maximum from any field type that can be compared, though that's of less importance.