Support emulator connections with sas tokens

[perfectayush]

Creating a draft request to address #310. This modifies Client::from_connection_string function to create development storage clients access with SAS tokens.

Example usage:

fn abs_emulator_client_sas(sas_token: &str) -> Result<KeyClient,AzureError> {
    let connection_string = format!("DefaultEndpointsProtocol=http; \
                                     AccountName=devstoreaccount1; \
                                     BlobEndpoint=http://127.0.0.1:10000/devstoreaccount1; \
                                     TableEndpoint=http://127.0.0.1:10002/devstoreaccount1; \
                                     UseDevelopmentStorage=true; \
                                     SharedAccessSignature=?{};",
                                     sas_token);
    return Client::from_connection_string(&connection_string)
}

This code is tested against Azurite. The SAS token client is able to list objects if it is correctly generated. Have to prefix SAS token with ? in connection_string for now as it is parsed with url::Url::query_pairs in get_sas_token_parms

[MindFlavor]

@perfectayush: please note we are migrating this repo here: https://github.com/Azure/azure-sdk-for-rust so I have to close this. Please feel free to reopen this PR at the new repo!