Added an XPC service to deal with sandboxing.

This fixes #163. It requires that you copy the XPC service into your main application bundle in
Contents/XPCServices and runs the remove from quarantine step of copying the finish_installation
app as well as launching finish_installation app outside of the sandbox. Sparkle will function
exactly as it does now for applications that do not copy the XPC service into their application
bundle.
Made XPC service calls asynchronous
Put networking code in XPC, too. Minor compiler fixes.
Removed additional log statements
Added limited support for remote release notes. Images / links / style sheets in notes won't work.
Cancel download no longer crashes the app.
Merged with master.
Refactored download XPC service: used NSURLDownload instead of lib curl; corrected behavior of SUXPCURLDownload class to be more common with NSURLDownload.
[Installation XPC Service] Added xpc message key constants instead of hard-coded string keys. Fixed memory leaks.
[Downloading XPC service] Added support of additional delegate method.
Added sandbox check.
Corrected using of macro.
Adds removing old data before downloading new one via XPC.
Updated xlc services configs.
Follow up for rev. [67ebc6f]
Made download service callback messages with sync reply to make sure that main app receive it.
Leave some connection leaks to avoid crash while closing it.
Updated products archive.
Merge branch 'sandboxing'