ARGO-337:  add support for tag normalization and validation

Contentable#decomission no longer defaults its parameter to nil

Identifiable:
-add helper methods:  split_tag_to_arr, normalize_tag_arr, normalize_tag, validate_and_normalize_tag
-add_tag, remove_tag, and update_tag now normalize when editing and comparing tag information
-new validation rules:  tags must not contain empty parts, tags must have at least two parts, tags are only considered the same if the whole thing is the same (not just the prefixes)

fix unit tests so that the test updated behavior.
ARGO-337: comments and a unit test rename
Move comments in Dor::Identifiable so that rubydoc parsers can pick them up
Use #detect to find and act upon a condition within an array
ARGO-337: add unit tests for validate_and_normalize_tag
ARGO-337:  fix duplicate tag checking
ARGO-337:  fix dupe checking in tag validation to be case insensitive
ARGO-337: add explanatory comment for tag dupe checking in validate_and_normalize_tag
Merge pull request #14 from sul-dlss/tag_normalization

Tag normalization