resolves #522 upgrade asciidoctor

This might provide a starting point for #522 (closed). After the obvious upgrades to use asciidoctor in its new location, and some method name changes, several tests fail.

Most of these are due to apparently changed include level tracking in asciidoctor. Fortunately there's a useful method to find out if the max depth is exceeded. The relative level doesn't seem to be the same however. I also don't understand why we wouldn't always report an error when the max depth is exceeded, nor how the condition for reporting could fail to hold.

At this point 5 tests in load-asciidoc-test.js fail:

• Missing block handlers no longer cause a warning. I suspect there's an info message logged, but we can't see it; adding logging should help.
• The output for two include nesting tests has changed; I don't know that we'd need to replicate the old output.
• Two footnote tests produce invalid html with the previous input, which looks exceedingly peculiar to me. Removing the escape on the final ']' makes the tests pass.

I've commented the significant change in the include processor and the test changes.