Support for data source abstraction

ImgLib provides an interface-driven mechanism for delivering image data from a particular source. For example, implementations might provide access to image data from:
- [x] files
- [ ] URLs
- [ ] an external system such as an OMERO database

Bio-Formats also has this type of data source abstraction, and it is worth investigating how and where this feature should be modeled in the new ImageJ API.

Regardless of implementation details, it would be nice to have at least one concrete example in code demonstrating access to image data from a source other than a file on disk.

Migrated-From: http://trac.imagej.net/ticket/21

Start by reviewing the new ImageJ API and the existing ImgLib file-based data-source interface, then compare how Bio-Formats models data sources. Determine where the abstraction belongs and identify a concrete non-file example, such as URL or OMERO access. Done means the API design is settled and one working example demonstrates image access from a non-file source.

Written by the indexing model from the issue text.