Added several new utility classes for Img/Labeling

Several new utility classes for Img/ImgPlus and Labeling were added.
Additionally a the new interface WrappedImg was added.

WrappedImg is an interface which can be used to retrieve the underlying
Img in e.g. a ImgPlus, as you need access to the Img to get detailed
information about it (e.g. is it an array img etc).

The helper classes ImgView, ImgPlusView and LabelingView allow one to wrap
an arbitrary RandomAccessibleInterval which than acts as an
Img/ImgPlus/Labeling.
ImgView: fix javadoc
WrappedImg: add missing license header
WrappedImg: add missing javadoc
WrappedImg: do not extend Img

Instead, WrappedImg is a "duck type" style interface that provides a
single method for unwrapping the Img in question.
Remove unnecessary ImgPlusView class

The following are equivalent:

    new ImgPlusView(raInterval, imgFactory, metadata);

with:

    new ImgPlus(new ImgView(raInterval, imgFactory), metadata);

It's slightly longer, but still one line of code. So the gain in
succinctness with ImgPlusView was minimal, and we decided not needed.
LabelingView: tweak javadoc
Merge pull request #43 from imagej/wrappedimg

Added several new utility classes for Img/ImgPlus/Labeling