Add logisim 2.7.1

An educational tool for designing and simulating digital logic circuits,
featuring a simple-to-learn interface, hierarchical circuits, wire
bundles, and a large component library. As a Java application, it can
run on many platforms.

http://sourceforge.net/projects/circuit/
Merge pull request #1850 from Proghat/logisim

Add logisim 2.7.1