Handle missing sync-back-ok

If a user skips syncback, the file sync-back-ok is never created. If it's
missing, skip checking each source separately in check_syncback_settings.

Fixes #158
Handle missing sync-back-ok in sup-sync-back-maildir

The script sup-sync-back-maildir was also not handling a missing sync file
gracefully.
make the source list whitespace-safe

It's possible to get some whitespace problems when manipulating the content
of sup-sync-back-maildir manually. This makes sure all whitespace is really
removed.
set script-encoding to utf-8 and use local dir for lib before search system
List whether source is usual, notify user of special handling

also list :usual as attr_reader in Source
fix elseif: delete sync-back-ok if sync_back_to_maildir has been set to false
Merge pull request #1 from gauteh/missing_sync_back_ok

Notify user of special handling for unsual sources and list whether source is usual
Merge #158: Maildir sync crashes when .sup/sync-back-ok does not exist

Conflicts:
	lib/sup.rb