Use diff thresholds for domain, top-level domain

Use a higher threshold when checking for an exact match with an entire
domain. This is useful because otherwise we have a problem where
corrections can "chain" - test@yahooo.cmo leads to a suggestion of
test@yahooo.com, which leads to a suggestion of yahoo.com.
Merge pull request #84 from jdpopkin/master

Use diff thresholds for domain, top-level domain