padded histogram array

- sending padded array (i.e. dates w/o stories are inserted to avoid a
“sparse array”) - see also here for discussion:
http://markmail.org/thread/szadjetlbx4xd2np
- fixed some imprecision in the way I count dates ($since - 1)
- changed to using json:array rather than map:map because it keeps the
order of elements (although thinking of it, a sequence of maps should
work just as well)
histogram code

- (very) basic html page to display the chart
- basic highcharts configuration; note the use of the API call in line
48
rxq-rewriter updated

to reflect my own pass-through paths.

also made a change to rxq.xqy to either return 200 or 404, depending on
whether the document exists or not.