Add support of ARC to Objective-C

Objective-C in Xcode 4.2 supports ARC (Automatic Reference Counting, not GC but similar) and now it is the default both on OS X Lion and iOS 5. (ARC works on iOS 4.2 or higher.)

The conventional Objective-C code includes many retain/release/autorelease for maintaining the life time of objects.

Since the latest Objective-C compiler automatically generates stubs for reference counting, code should not include any retain/release/autorelease.

Many Mac OS/iOS projects are moving to ARC mode. Although we can specify a compiler flag which tells ARC or no-ARC file by file in Xcode 4.2, there is no means to tell the Xcode that files generated by the thrift compiler are no-ARC. Xcode produces many error messages like "retain is not allowed in ARC mode".
So, the thrift compiler should support ARC mode.

What we need are:

1) add -objc-arc flag to the thrift compiler.
2) If -objc-arc flag exists, omit generating code for retain/release/autorelease.

Please look at "Programming with ARC reference notes" in iOS developer library.

http://andpdas.com/wp-content/uploads/2011/06/ARCProgrammingGuide.pdf