Be able to parse numbers with no digits before the decimal point.

Per the [SVG spec](http://www.w3.org/TR/SVG11/types.html#DataTypeNumber), non-integer number values do not necessarily have any digits before the decimal point.
Be able to parse viewbox numbers with no digits before the decimal.
Add test for parsing numbers with no digits before the decimal point.
Merge pull request #1317 from rodovich/leading-decimal

Be able to parse numbers with no digits before the decimal point.