Allows user to set the allowsCellularAccess

This patch allows disable cellular access per request.

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)