added code to find ebook-convert

Ran into a few issues generating the mobi file.

I copied code from ebook:check to makeebook, so when the check passed, it properly generated the mobi file for me.


If you don't want the duplicate code, please advise a direction, and I can revise pull request to try again.

The only alternative I saw was to convert makeebook into a ruby script/class and leverage from the rake task ebook:check. But didn't want to suggest that route if you thought it introduced too much change.


(On a side, do you suggest using this workflow for other publishers, or would you advise a different path?)

Thanks again for this example.
Keenan