refactor api to remember node settings #10

#10

## Changes

Summary:

 * **Change setting for new nodes**<br/>New nodes use the active comment source for the node type.<br/> <br/>
 * **Override a node to off**<br/>If a user selects a comment source, and then turns off comments for a specific node, future changes to the active source will not affect this node.<br/> <br/>
 * **Nodes remember comments by default**<br/>If a node gets comments from the active source, then the user changes the active source, future changes to the active source will not affect this node. BUT, the user can always switch to hide existing comments and use the newer comment setting.<br/> <br/>
 * **Automatic switch for node without comments**<br/>Nodes without any comments (that have not been overridden) will automatically load up the active comment source for it's node type.