XMPPCapabilities should update c element if present

Presence stanzas sent by XMPPStream's resendMyPresence may already
contain a c element. If a c element is present in the presence stanza
XMPPCapabilities should replace it rather than add an additional c
element.
XMPPvCardAvatarModule should update x element if present
Remove all uses of dispatch_get_current_queue, except for logging.

All uses of dispatch_get_current_queue (deprecated since iOS 6.0) are removed,
and replaced by assigning a static queue tag, which is checked for its value.
Inside the queue, the specific will return itself, while outside the queue it
will return NULL, which will make the check fail.

Only two instances of dispatch_get_current_queue remain, both have been
surrounded by pragmas to disable the warning by the Clang compiler.
- DDLog.m: obtaining the current queue label.
- DelegateTesters.m: obtaining the current queue label for NSLog.
Merge branch 'remove-get-current-queue' of https://github.com/drodriguez/XMPPFramework into drodriguez-remove-get-current-queue

Resolved Conflicts:
	Utilities/XMPPIDTracker.m
	Vendor/CocoaAsyncSocket/GCDAsyncSocket.h
	Vendor/CocoaAsyncSocket/GCDAsyncSocket.m
	Vendor/CocoaLumberjack/DDAbstractDatabaseLogger.m
	Vendor/CocoaLumberjack/DDFileLogger.m
	Vendor/CocoaLumberjack/DDLog.m
	Vendor/CocoaLumberjack/DDTTYLogger.m
Converting queueTags from 'char *' to 'void *', and removing unneeded comparisons
Adding all files to Xcode project in order to test for warnings throughout framework
Bug fix for issue #156 : Bug in XMPPRoomCoreDataStorage causing duplicate messages
Merge pull request #121 from andrewtj/atj-dedupe-presence-elements

XMPPCapabilities should update c element if present