resolves #29 add configuration for loading Ruby-based extensions

- allow GEM_PATH to be specified using --gem-path option
- allow requires to be specified using -r option
- fix spellings of AsciiDoc
- remove a few dead imports and options
Watch out when you split on "comma", Maven's javadoc plugin splits the `<additionalparam>` element's value on comma before passing args to the doclet (no idea why, but it does) so that will break javadoc's option parsing. That's why I used semicolon for attributes.

You could also use multiple occurrences of the "-r" option. I didn't think javadoc supported that, but a quick test shows that it can. In hindsight that might have been better for attributes as well.