When a user issues "bookshop build mobi", commands.rb should call executables from within the bookshop gem rather than calling them from the bookshop project /script folder.
So the kindlegen, epubcheck, zip-info, and any other executables should reside within the bookshop gem. They should not be copied into each newly generated project.
This will ensure that the executables compatible with the correct version of bookshop are used.
see this article http://stackoverflow.com/questions/10656348/in-a-cli-gem-where-do-i-store-third-party-binary-executables
finished moving binaries over to bin path for gem, closes #44