Proper apostrophes in default validation messages

The important change is the one in `activemodel/lib/active_model/locale/en.yml`.  The rest are all just updates in tests and docs.

The default validation messages with contractions (“can’t be blank”, “can’t be empty”, “doesn’t match [attribute]”) should use real apostrophes (_i.e._, smart/curly quotes, like `’`) instead of straight quotes (like `'`).  Straight quotes are a relic of typewriters and ASCII.

Since these errors are ultimately user-facing strings, they should be nice looking ones.

See http://smartquotesforsmartpeople.com for some more straight talk about straight quotes.