Adds WebDAV file sync

- Still experimental and incomplete, with no lock support and not much error handling

Also:

- New expiry date for sync functions
- Attachment character set was being dropped during syncing
- Possibly improves sizing issues with preferences window
- Fixes problems with attachment filenames with extended characters
- Fixes some problem with tags that I don't remember
- Makes XMLHTTPRequest calls are now background requests (no auth windows or other prompts)
- Z.U.HTTP.doOptions() now takes an nsIURI instead of a URL spec
- New methods:
  - Zotero.Utilities.rand(min, max)
  - Zotero.Utilities.probability(x)
  - Zotero.Utilities.Base64.encode(str) and decode(str)
  - Zotero.getTempDirectory()
  - Zotero.Date.dateToISO(date) - convert JS Date object to ISO 8601 UTC date/time
  - Zotero.Date.isoToDate(isoDate) - convert an ISO 8601 UTC date/time to a JS Date object

components/zotero-service.js

@@ -51,6 +51,7 @@ var xpcomFiles = [
 	'schema',
 	'search',
 	'sync',
+	'storage',
 	'timeline',
 	'translate',
 	'utilities',