debug mode for spacefinder

- small fix for custom selector checks (see line 30 - all tests should be from the insertion point, e.g. the top of the paragraph)
- added debug argument that annotates paragraphs with classes/attributes to expose debug information (to be used with the bookmarklet [here](https://gist.github.com/mattosborn/c6cc61a69bb049b45446))

Screenshot
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![image](https://cloud.githubusercontent.com/assets/960919/5377148/5a97095a-8076-11e4-9f55-2fb6af79f1b0.png)

// @ironsidevsquincy 
in general, this will probably be the one that fails - can we get any more information in here, i.e. which element exactly it failed on?