Fixed bug in pie chart for values summing to 0

When values are given and all of them are 0 a divison results in a NaN value
that causes an invalid attribute value. Checked whether all values sums to 0 in
the pie directive and added an example in the index file.
Centered text vertically in pie chart

The text is now vertically centered in the circle. Did also ajust the thickness
of the circle (now 25% of radius).
Merge pull request #4 from bdo-labs/fix-pie-chart

Fix pie chart