Check if node is running

It would be useful to expose a function to check if the node is running (ie `node.start()` ) was invoked. 

A possible solution:
```rust
/// Check if node runtime is up
pub fn is_node_running(&self) -> bool {
 if self.runtime.read().unwrap().is_some() {
 true
 }
 else { false }
}
```

