Allow Range and Content-Range headers to contain any valid range unit

- Range units other than 'bytes' may be provided as part of a Range
  or Content-Range header
- Updated __repr__() tests for Range and ContentRange to actually test
  the return value of the repr call (previous test was only asserting
  that __repr__() returned a 'truthy' value)
- Updated Range repr to return a string representation enclosed in angle
  brackets, matching the style of most other WebOb classes
  (vs. a string that may be able to be passed to `eval`)
- Fixes #177