Made the HTTP header matching case insensitive

I've noticed that some servers return ETag headers with name "Etag" or "ETag" or "ETAG", etc. which does not get matched by NSDictionary's "objectForKey" method. HTTP header names are case insensitive. This commit allows case insensitive search.
Fix a bug if Image decompression failed, no callback for the operation finish
Merge remote-tracking branch 'trawor/master'
Merge remote-tracking branch 'cbrauchli/caseInsensitiveDictionary'

Conflicts:
	MKNetworkKit-iOS/MKNetworkKit-iOS.xcodeproj/project.pbxproj
	MKNetworkKit/MKNetworkOperation.m