IM3Reader for Perkin-Elmer Nuance

This is a basic reader for the Perkin-Elmer Nuance .im3 format. I don't have access to the file format spec, but I have a fair degree of confidence that the reader works. There isn't much in terms of metadata extraction.
```ENCODING``` in the ```loci.common.Constants``` class is recommended instead of ```Charset.forName("UTF-8")```.  The ```readString(int)``` method can also be used to simplify this if using a ```RandomAccessInputStream``` instead of ```IRandomAccess```.