Add implementation of Plurk service.
Fix compiling error for Plurk service.
Add Plurk demo app API key.
Better error handling and ask user to relogin when required.
Rewrite image uploading response handling using JSON.
Merge remote-tracking branch 'dlackty/plurk-service' into plurk

Conflicts:
	Classes/Example/ShareKitDemoConfigurator.m
	Classes/ShareKit/Configuration/DefaultSHKConfigurator.h
	Classes/ShareKit/Configuration/DefaultSHKConfigurator.m
	Classes/ShareKit/Core/SHKSharers.plist
	ShareKit.xcodeproj/project.pbxproj
BUGFIX: added target for Plurk, removed SBJson use
BUGFIX: URL and Image were not shared on plurk
REFACTORING: SHKSharer updated to modern syntax
REFACTORING: shorten URL is now handled by framework + Plurk uses builtin OAuth method to share binary image