Improve ActsAsXapian::Search#words_to_highlight

- Ignore stopwords
- Use Stemming
- Wrap words with regex for use in a highlighter (optional)
Add helper to highlight and excerpt by regex

Backport of https://github.com/rails/rails/pull/11793/

Contains integration tests to check that it works
as expected with ActsAsXapian.
Use regex based highlighting
Add spec for highlight_and_excerpt
Handle unhelpful stemming

Stemming returns 'bore' as the word to highlight which can't be matched
in the original phrase.

Also removes duplicates from the results
Merge branch 'issues/1434-email-highlighting' into rails-3-develop