fix permission issue on some environment

[hardywu]

unzip the wkhtmltopdf binary on installation using gem extension.
related #57

[unixmonkey]

Interesting, setting up an empty c-extension to unpack the right binary on install.

Could this be accomplished with a simpler post_install task like so:

Gem.post_install do
  # unzip correct binary
end

Or does that still have the permission issue?

[hardywu]

@unixmonkey No, It can't. post_install hooks are not working at the level within the gem package.