Disable comment related XML_RPC methods

Replaces comment related XML_RPC methods with a placeholder function.

Unfortunately but understandably the WordPress iOS app does not check if methods exist before showing their respective UI. Rather than just removing the XML_RPC methods altogether the placeholder function returns a more friendly error message.

![nicer-error-message](https://f.cloud.github.com/assets/4179791/1181660/ae5e5530-220c-11e3-8e6a-bd4201901589.png)