Use Asciidoctor 1.5.0 new syntax backtick

Asciidoctor 1.5.0 is now using backtick for inline code.
Replaces ASCIIDoc by AsciiDoc to respect the case.
Adds headers to the AsciiDoc editor
Merge pull request #862 from Mogztter/asciidoctor-1.5.0-syntax

Use Asciidoctor 1.5.0 new syntax backtick
Merge branch 'master' into rc
Merge pull request #864 from Mogztter/asciidoc-headers

Adds headers to the AsciiDoc editor
Made the Gollum theme responsive.

Not a particularly comprehensive change in style, just one that removes all the fixed sizing for browsers below 940px in width.

Closes #831.
Horizontal scroll when table is too wide
Fix css selector for Asciidoctor h1 html ouptput
Merge pull request #873 from Mogztter/issue-871

Fix css selector for Asciidoctor h1 html ouptput
Merge pull request #872 from Mogztter/issue-870

Resolves #870.
allow uploading files by drag and drop
Merge pull request #869 from lucas-clemente/rc

allow uploading files by drag and drop (if uploading files is enabled).
Fix the commit information for page deletion.

Now they will be authored by the gollum.author.
Merge pull request #876 from LEW21/delete-commit-rc

Fix the commit information for page deletion. Solves #807.
add detail to inline help
Merge pull request #884 from tuftedocelot/issue600-docs

add detail to inline help
Page header now uses h1-title logic.
Merge pull request #889 from repotag/888-h1-logic

Page header now uses h1-title logic. Resolves #888.
Do not render footers, headers and sidebars if (after formatting) they are empty. Resolves #398 .
Change pages display to handle non-wiki file types with --show_all enabled