use the defaultWidthX, not the nominalWidthX
Merge pull request #56 from mapbox/defaultwidthx

Use the defaultWidthX as the starting value.

Previously, we used the nominalWidthX; however, this value should only be used when a width is specified in the charstring. Then the nominalWidthX is added to the glyph width. However, if no width is specified, we should use the defaultWidthX.