The user experience when using sql-mode is somewhat lacking. In  
particular, the behaviors when using a SQLi buffer could be more  
helpful.

The attached patch:
  - Adds a default binding (C-c C-t) to sql-set-sqli-buffer
  - Directs the user to the function if they attempt to call sql-send- 
* with an unusable sql-buffer.
  - Updates sql-help to mention sql-set-sqli-buffer.