Fix access to FrontMatter on Middleman >= 3.1

- Access to extension has changed from `.frontmatter_manager` to
  `.extensions[:frontmatter]` in Middleman >= 3.1. The fix is backwards
  compatible.
- Bumped up gem version from 0.0.2 to 0.0.3
Fix bug in accessing @app in BlogDataExtentions

Not sure if this change is backwards compatible, bumping up dependency
on middleman-core to 3.1.4
Indent code
Fix Gemfile source
Check in Gemfile.lock
Add an option to force drafts in build mode

This comes in handy in e.g. staging environments.
Add dependencies required to run specs with new middleman version
Fix specs and update readme
We are only supporting 3.1+ now, lets not worry about 3.0
Travis!
Tweak gemfile as an attempt to fix the build
v0.1.0
Add a configuration section to README.md

Add a configuration section to the README. Documents the `build` option
and gives a short example oh how to use it.

Also, amend the `listing` section with a piece of code that behaves more
consistently with the behaviour of draft building.