Added preprocessor check for iOS8 extensions

[fraserscottmorrison]

Ive done a fix for this bug #438

Got MKNetworkKit working in a Share Extension. A new preprocessor macro TARGET_IS_EXTENSION is needed to avoid calls to [UIApplication sharedApplication]

To set up with cocoa pods was a little tricky, on 'pod install' it'll create a separate pod target and add the preprocessor macro - podfile looks like this

link_with 'App', 'Extension'

pod 'MKNetworkKit'

target 'Extension' do
    post_install do |installer_representation|
        installer_representation.project.targets.each do |target|
            if target.name == 'Pods-Extension-MKNetworkKit'
                target.build_configurations.each do |config|
                    config.build_settings['GCC_PREPROCESSOR_DEFINITIONS'] ||= ['$(inherited)', 'TARGET_IS_EXTENSION=1']
                end
            end
        end
    end
end

[MugunthKumar]

Thanks!