Large images

This breaks the 2GB width*height*slices limitation, which results in a `NegativeArraySizeException` as [reported on #fiji-devel](http://code.imagej.net/chatlogs/fiji-devel?search=&start-date=2014-04-22&end-date=2014-04-22&format=html&columns=78&times=sparse&submit=search):
```
java.lang.NegativeArraySizeException
        at util.FindConnectedRegions.run(FindConnectedRegions.java:347)
        at util.Find_Connected_Regions.run(Find_Connected_Regions.java:105)
        at ij.IJ.runUserPlugIn(IJ.java:199)
        at ij.IJ.runPlugIn(IJ.java:163)
        at ij.Executer.runCommand(Executer.java:131)
        at ij.Executer.run(Executer.java:64)
        at java.lang.Thread.run(Thread.java:662)
```
@mhl, @dscho: See anything blatantly wrong? Main relevant commits are e62eec4 and 0ac8da3. Unit tests still pass. But I don't have enough RAM on my box to test it with a really large stack.