Add a bulge correction factor to the outer contour to make outer dimensions accurate

Yet another dimension correction patch. This one doesn't address filament stretching for rounded contours, but merely adjusts for the bulging of the outer surface of the outer perimeter. Assuming a circular bulge profile, the correction factor is easy to calculate and no parameter is needed.

Here is an example of how, with this patch, a 10x10 mm square peg (with rounded corners) fits tightly into a 10x10 mm square hole with quite good dimensional accuracy:

![h_id](https://cloud.githubusercontent.com/assets/3725313/3875737/ffc3dfd8-2154-11e4-8612-d4560e9d9162.jpeg)
![h_od](https://cloud.githubusercontent.com/assets/3725313/3875741/ffd49594-2154-11e4-9f61-395b4cd4f748.jpeg)
![p_od](https://cloud.githubusercontent.com/assets/3725313/3875740/ffc6b334-2154-11e4-9fbb-b761a44889f3.jpeg)
![fit2](https://cloud.githubusercontent.com/assets/3725313/3875738/ffc44b6c-2154-11e4-9bcd-32104285f371.jpeg)
![fit1](https://cloud.githubusercontent.com/assets/3725313/3875739/ffc5d388-2154-11e4-90ff-1edb46ab560a.jpeg)