Thanks for making this tool. I have been trying a few imports but am having a few issues.
After trying your script I was able to get it to work, but the imported terrain seems to have some issue with breaks in the terrain. It is as if the heightmap was only applied to every other 'line' of terrain. I am not sure what is causing this. I tried altering the terrain size to be half as large, but your script doesn't allow for the terrain and heightmap to be incorrectly proportioned (which I think is a good thing assuming it works correctly).
Here is a link to what the profile of the terrain looks like:
http://imgur.com/OuQ8gvgEdit: I think it is worth noting that this happened with the following three heightmaps I tried:
http://imgur.com/a/ICmU6#0