AsciiDoc template language support

This commit includes [AsciiDoc](http://asciidoctor.org/) support into Sinatra.
AsciiDoc support was merged into Tilt with [this commit](https://github.com/rtomayko/tilt/commit/0485f920fc63a9c73932cb36c95580292b1c5363).

Batteries included — tests, documentation in English and Russian, Gemfile update.