Allow Donut to handle PHP json_encode output

Currently, Morris.Donut produces a chart with a load of tiny little bars on the bottom right of donut charts generated from data straight from PHP's json_encode function.

The other chart types don't do this, because they parse the values into Int/Float values.

This change adds "parseFloat" to the line of the Donut code which reads the x.value, which brings it into line with the other chart methods, and makes json_encode output work as expected

It also has a longer commit summary than the amount of code added...