Stretch algorithm

This is a patch I used to fix incorrect diameters of vertical holes and, in general, insufficient inner dimensions. I created a new parameter "stretchDistance". When this parameter is zero (the default), nothing new occurs. When the parmeter is 600 - for my printer - the system increases diameters in a manner all diameters are accurate, even small ones.
The idea behind the algorithm is that the command sent to the printer is like the position of the front wheel of a bike, and the real position of the plastic is the position of the rear wheel. stretchDistance is the distance, in microns, between the front and the rear wheel. Maths : http://www.mathcurve.com/courbes2d/tractrice/tractoiredecercle.shtml

The following pictures show the effect of the stretch algorithm. A simple object with 1, 2, 3, 4, 5, 6, 7 and 8 mm holes diameter is a perfect fit for corresponding drills
![stretch2](https://f.cloud.github.com/assets/6553898/2240531/64a972d2-9c99-11e3-8559-0a7af74ca3ef.jpg)
![stretch1](https://f.cloud.github.com/assets/6553898/2240532/6a8ce846-9c99-11e3-9ec6-1863097d8eda.jpg)

Previous (closed) pull requests are #53 and #54