Log benchmark of PDF generation.

A couple of notes here:
* Didn't know how to test this since I have no access in the internals of `Rack::Lint::ErrorWrapper` but it will itself test the API of the logger is correctly used.
* Is `require 'benchmark'` ok there or should it be in `lib/pdfkit.rb`?
* Some other middlewares accept a logger in initialize and use that when given instead of `rack.errors` but I was personally not interested in that and it didn't seem that much useful to bother.