Use go-bindata from github.com/jteeuwen

It is now able to recursively walk into directories and is also able to
generate the debug version of the assets. Therefore, we remove the
manual version of the debug version and use the generated one instead.

The option to choose the name of the function (`-func`) was available at some point but not available anymore, hence the renaming to Assets which could be avoided.

Also, you may be wondering why I keep coming with pull requests like this one: I am trying to package ngrok for Debian and I have to package all dependencies. Therefore, I prefer to package the non-forked version if possible.