Adds one possible realization of a SLAB (Standoff Layered Annotated Blob). Slightly extended from my previous proposal so that SLAB can be parameterized by content type (allowing SLAB[String,...], SLAB[Image,...], etc.).
Merge pull request #17 from bethard/master

Here's one possible realization of a SLAB (Standoff Layered Annotated Blob)