Concept of coverage, last modified etc in data package

[rufuspollock]

There are certain concepts that people use regularly that aren't in core schema but could be a pattern so people have a convention e.g.

• modified / last modified (and last modified for data vs metadata ...) - note we removed this from core in [DP] modified/last_modified is confusing datapackage#83
• coverage (e.g. temporal or geographic) - cf http://www.dublincore.org/specifications/dublin-core/dcmi-terms/#terms-coverage