IE packaging fixes

This patch does three things
- On OSX use the NSIS binaries included in this repository to build he IE installer
- Only require `signtool` if a certificate was actually specified in the config
- Use `osslsigncode` binaries included in this repository on OSX and Linux (commented out)

The last item is commented out because `osslsigncode_osx` is currently incorrectly linked against `/opt/local/lib/libcrypto.1.0.0.dylib` which is not part of OSX. Presumably it was built while some third party package manager like homebrew was interfering. The corresponding system library is `/usr/lib/libcrypto.dylib` and `osslsigncode_osx` should be rebuilt using it.