Support for ARC and error reporting

[tomaz]

Hey! I've adopted XPCKit to my app and it's been really smooth sailing so far (much smoother then using XPC API directly, not to mention clearer code etc). Gladly I've code across your lib as I was about to write my own (not nearly as sophisticated, just to cover my needs). Anyway, I added two things which I use in my app:

• ARC support: works for both non-arc and arc builds, although it makes the code a bit ugly due to #if/#else statements. If you later port it fully to ARC, you can remove all the conditionals (probably makes sense as XPC itself is Lion only, so probably folks will also adopt ARC).
• Error reporting support: there's additional block handler in XPCConnection that's invoked when an error is detected. Also updated TestApp to demonstrate how it can be used.

If you think this would be useful for other users, feel free to merge into main repo, otherwise just close the pull request :)

Thanks for the lib! Tom

[avaidyam]

Instead of the if/else preprocessor directives, consider macros like this:

#if  __has_feature(objc_arc)
#define OBJECT_AUTORELEASE(object) (object)
#elseif
#define OBJECT_AUTORELEASE(object) [(object) autorelease]
#endif

... and so on. This way, one macro controls everything and can be modified accordingly.