Add PortPlacement extension

This extension assists in the planning of surgical port placement in a laparoscopic procedure. Additional details at http://www.slicer.org/slicerWiki/index.php/Documentation/Nightly/Modules/PortPlacement
Fixed several URLs in PortPlacement extension description file
Changed module category to IGT. Update git revision to new version with bugfixes
Merge pull request #367 from giogadi/add-PortPlacement

Add PortPlacement extension