Better metadata classes

The `net.imglib2.meta` package and related classes were all marked deprecated, but without any replacements given. Furthermore, pursuant to #43, we must have a single package prefix to be OSGi friendly.

This branch creates replacement classes for most of the old `net.imglib2.meta` classes. It updates all `net.imagej` code to eschew the use of deprecated `net.imglib2` classes from this same component, in favor of using the new classes (mostly in `net.imagej.axis`, `net.imagej.space` and `net.imagej.interval`).

It creates temporary replacements for `ImgPlus` and a couple of its related interfaces, so that we can fully ditch the old `imglib2-meta` classes without waiting for a reengineered and unified `ImgPlus`+`Dataset` data structure.