FEATURE: Helpful methods for handling ETags and Last-Modified-Since;
  Just implements single or multiple etags, but not the "*" glob
  format or other esoteric schemes never seen in the wild.

QA: None; this will be integrated into other componets and QA'd as
  part of other projects.

Reviewed-by: dan.rathbone@socrata.com
Bump version to 1.3.1