Metric Histogram Plot

Update to the histogram plot to now plot long term metrics.
It enables you to plot distribution of say, Intensity Factor
for a season or cycle.

You can select the y-axis metric too, so rather than just
plotting duration you could plot say, TSS accumulated for
different ride intensities.

Fixes #560
Enabled progress translation in PowerTap download
Updated spanish translation
Notes (<cmt/>) from PWX files not added as RideFile tags...
Merge branch 'master' of github.com:GoldenCheetah/GoldenCheetah