Merge lp://staging/~jml/launchpad/split-mail-and-summary into lp://staging/launchpad

Improve the email sent by ec2 test.

Hello,

This branch changes the email sent by ec2 test. Rather than being a mere lump of stuff that we have collected as summary, it's a custom built message derived from the test results.

Examples:
  * http://paste.ubuntu.com/497557/
  * http://paste.ubuntu.com/497555/

Implementation-wise, the main trick is to change EC2Tester, the object responsible for running tests and gathering results, to send its TestResult object back to the logger, rather than a simple boolean. The Logger was then changed to use the new format_result method to compose the result email, rather than simply dumping the summary.

The details:
  * If the test child process exits with a non-zero code and we have no test results, we raise an error, since it's likely to be something weird.

  * Included in the diff are changes from a merge proposal that I should have applied before landing my previous branch. Mostly stylistic.

  * Some tests were incorrect. They used a dictionary to fake an email message, but had the subject stored under the wrong key ("Subject:" rather than "Subject").

I look forward to your review.

jml