fix for iframe insertion

Related to change from here: https://github.com/code-dot-org/blockly/pull/661

The blocks in the message were broken, looking like this:
![screen shot 2014-06-12 at 9 53 06 am](https://cloud.githubusercontent.com/assets/5429146/3260826/10154a5c-f252-11e3-8fe5-941549f78359.png)

instead of like this (the way they used to be):
![screen shot 2014-06-12 at 9 51 37 am](https://cloud.githubusercontent.com/assets/5429146/3260811/ef37b37e-f251-11e3-952a-5f70213b2c7f.png)

I believe the intention was to insert the feedbackBlocks div into the feedback message paragraph for the new hint style case, so I've tried to narrow the code to do it only for this case.