Add Jekyll to Gemfile as optional (#741)

This will allow local preview of docs/ with:

    bundle install --with=docs
    bundle exec jekyll serve --source docs

.gitignore

@@ -11,3 +11,5 @@ pkg
 .dev/
 .bundle/
 vendor/
+_site/
+docs/.jekyll-cache/

Gemfile

@@ -3,3 +3,7 @@ source "https://rubygems.org"
 gemspec
 
 gem "activeresource", "~> 5.1"
+
+group :docs, optional: true do
+  gem "jekyll"
+end