Add constants TILE_TOP/TILE_LEFT and variables tileTop/tileLeft

This avoids the ambiguity in the use of magic numbers to access
properties in a tile record by offset.
Add empty line
Add rect elements for the bounding box of each tile

While the circle element and the surrounding group only extended
to the surface of the circle, the bounding box rect covers the
whole tile, which will facilitate the selection of a tile to obtain
extra information about an author. Note that the parent g element now
covers the whole tile as well.
Wrap charts in a div to apply extra CSS styles (min-height)

The min-height applied to the SVG element with a "height"
attribute had no effect. I first tried to apply a minimum
height value, but this was not flexible enough since it
was applied to all types of charts. I then thought of
wrapping each chart in an extra div, with the class 'chart-box'
for all charts, and I moved the CSS class which reflects
the grouping from the SVG element to the parent div.
Merge branch 'enhancements'
Merge branch 'master' into gh-pages