Support labeling each point of an axis

The `net.imagej.axis` package should be updated to support labeling each sample along an axis. The central use case for this is for channel names, but it would be potentially useful for any dimension.

See also [this thread on imagej-devel](http://imagej.net/pipermail/imagej-devel/2015-May/002580.html).

Start by reading the net.imagej.axis package and the linked imagej-devel discussion to understand the proposed axis-labeling model. Define what support for labeling every sample must cover, including the channel-name use case and other dimensions; the work is done when the package can represent and use those labels consistently.

Written by the indexing model from the issue text.