Allows user to set the allowsCellularAccess

This patch allows disable cellular access per request. eg, use WIFI only.

It sets whether the connection can use the device’s cellular radio (if present). default: true.

    BW::HTTP.get('http://example.com', {allows_cellular_access: false}) do |response|
        ...
    end

background of this option can be found [here](https://developer.apple.com/library/ios/documentation/NetworkingInternetWeb/Conceptual/NetworkingOverview/Platform-SpecificNetworkingTechnologies/Platform-SpecificNetworkingTechnologies.html#//apple_ref/doc/uid/TP40010220-CH212-SW9)