Refactor Hash.fetch usage to provide default values

Rather than explicitly running through if/else blocks, it's easier to
read if the default value is provided against the Hash method calls directly.
No need to store fetched values in local variables; use directly

These fetched values are easier to read when placed directly into the
Hash that they're going to be used in. No need for the temporary
variables to be used and means more of the class can be viewed at a time.
Merge pull request #443 from KushalP/email-idiomatic-variable-assignment

Refactor LogStash::Outputs::Email.register method
Merge git://github.com/logstash/logstash