Add Gemfile and Rakefile

This commit adds a Gemfile to simplify installing Jekyll and dependencies. It also provides a Rakefile for creating posts.

https://trello.com/c/EmXpKZQt/2172-simplify-apiary-blog-workflow
FTR, I really don't like the syntax that is natural for ruby work, but feel very alien for CLI. Would expect something like `rake post --with-images "Hello World"`

That said, definitely better than what we have ;)