Histogram Compare Metric

.. the last of the histogram updates to compare date ranges
   for distribution by metric (e.g. IF for rides)
Fix Histogram SEGV and Speed

.. SEGV caused by referencing data->isChecked when
   not in range mode

.. Speed plotting broken because line setting the
   array length in setData was accidentally deleted
using milliseconds for delta_t when calculating speed, supports Suunto Ambit gpx datafiles
Merge branch 'master' of git://github.com/GoldenCheetah/GoldenCheetah