broke out _defaultMessageIntro()

Broke out code for generating introduction text of build result
email from buildbot.status.mail.defaultMessage() function into
a new function.
broke out _defaultMessageProjects()

Broke out code for generating project list info for build result
email from buildbot.status.mail.defaultMessage() function into a
new function.
broke out _defaultMessageSourceStamps()

Broke out code for generating source stamps info for build result
email from buildbot.status.mail.defaultMessage() function into a
new function.
broke out _defaultMessageSummary()

Broke out code for generating summary text for a build result
email from buildbot.status.mail.defaultMessage() function into
a new function.
broke out _defaultMessageURLs()

Broke out code for adding URLs to a build result email
from buildbot.status.mail.defaultMessage() function into
a new function.
unit test on mail.defaultMessage() function

Added unit test on buildbot.status.mail.defaultMessage() function
Merge elmirjagudin/buildbot:mail-utests (PR #1039)