Research wkhtmltopdf alternatives

[jniles]

Bhima's PDF rendering is powered by the wkhtmltopdf library. So far, it has done the job satisfactorily; however, there seem to be some stability issues. My concerns are as follows:

1. npm install wkhtmltopdf doesn't always work in practice
2. The version of wkhtmltopdf bundled with some distros is buggy
3. Building from source is impractical for Congo --- requires massive downloads and is complicated for unsupported distributions

Perhaps there are no alternatives, but these issues make working with wkhtmltopdf quite difficult, especially on the variety of architectures and quality of networks that we run on.