master/status/mail: Use distinct filenames for the e-mail build summary

When buildSetSummary is set, use distinct filenames for the
attachments so it's clear which attachment belongs to which builder.
Support empty password for SVNPoller
Fix pep8 errors
Merge hwoarang/buildbot:master (PR #1121)

+autopep8
Merge santegoeds/buildbot:support-empty-password-svnpoller (PR #1122)