Update Variables for an email address in an address book

@param callback
@param id
@param email
@param variables
function updateEmailVariables(callback,id,email,variables){
    if ((id===undefined) || (email === undefined) || (variables === undefined) || (! variables.length)) {
        return callback(returnError("Empty email, variables or book id"));
    }
    var data = {
        email: email,
        variables: variables
    };
    sendRequest( 'addressbooks/' + id + '/emails/variable', 'POST', data, true, callback );
}