Fork from Slic3r master branch, including recent updates from alexrj.

Changes discussed in issue #1677 and pull request 208.

Features implemented:

• Pressure control
  • The main goal is to create a pressure control mechanism mainly for bowden setups, but it can be used in other setups, controlling the different pressure in the filament based on the extrusion speed. This way, the filament will be pushed or pulled a specified amount during speed changes to compensate the different pressure required, maintaining the correct extrusion flow.
  • Use the same logic as per Matt Roberts Advance Algorithm (the pressure is proportional to speed^2)
  • Triggered every speed change
  • A new extruder parameter let you define the advance constant
    • 0% disables the feature
    • 100% will push or pull 100% of the amount calculated by the algorithm.
    • Use this to adjust the extra amount of filament that the algorithm will insert in your gcode.
  • The extra amount in the system is accounted during retraction and unretraction, allowing the use of a smaller retract distance
• Unretract speed
  • Use a different unretraction speed from retraction (slower, for instance, to allow the melting in the hotend)
  • Zero will disable the feature and the retract speed will be used