Change the elmo-folder-synchronize use of MASK

Use the MASK argument, if supplied, to compare against the list of
messages in the msgdb to determine which messages need to be
synchronized.
Merge pull request #79 from egh/feature/use_mask

Change the elmo-folder-synchronize use of MASK
Add ChangeLog entry.
elmo-multi.el (elmo-folder-search): Rewritten.  Avoid redundant use of elmo-folder-list-messages.
Remove elmo-list-difference.