Create Product Design Trail
Add product-design cost notation
Expand product-design's MVP Acronym
Add community driven Ruby style guide
Add GitHub Ruby style guide
Merge branch 'mikepmunroe_edits'
Update book links to amazon affiliate shortened links.
Merge remote-tracking branch 'origin/pr/71'
Add symbol for paid resources
Merge branch 'pr/66'

Add Product Design trail
Convert trails from markdown to JSON

* The trail-map is now JSON-only for use at learn.thoughtbot.com
* All Pull Requests should update the JSON as per CONTRIBUTING.md

Notes on this commit:

Convert ruby trail to json
Convert code-review to json
Convert css trail to json
Convert git trail to json
Convert grids trail to json
Convert html trail to json
Convert information-design trail to json
Convert ios trail to json
Convert Javascript trail to JSON
Convert open-source trail to json
Add prerequisites to open-source trail
Convert postgres trail to json
Convert rails trail to json
Convert refactoring trail to json
Convert rubymotion trail to json
Convert sass trail to json
Convert test-driven-development trail to json
Convert typography trail to json
Convert unix trail to json
Convert vim trail to json
Convert visual-design trail to json
Lint json and resolve errors
Add descriptions to trails
Use name instead of title in ruby trail resources

Change resource json to match <a> tag properties
* url => uri
* name => title

Rename visual-design and postgres trails
* Visual Design => Design Principles
* Postgres => SQL
Fix typo in git trail
Fix capitalization of trail names
Refactor test-driven-development into testing trail

Remove markdown-style code ticks ("`")
* Code ticks are not currently being parsed in json, and as such
  currently distract from content rather than adding clarity.
* The parsed trail-maps on learn.thoughtbot.com/javascript, for
  example, look fine without <code> wrappers.
* If we miss these, we can always bring them back.

Simplify format of validations in trails

* Always use an array of strings as the validations for a step
* This is in keeping with the original intention of the json trail-map
  spec.
* Links can be added by including them in the validations string, but in
  general should not be used in validations and will likely fit better
  as a resource.

Fix formatting of ruby trail

* For some reason, there was an invalid "guides" element. This has been
  converted into a step
Update docs to reflect new JSON scheme

* Update `README.md` to include a notice of new direction
* Update `CONTRIBUTING.md` with JSON info and linting before commiting
* Update links to learn.thoughtbot or json file
* Add gist for RegEx
* Update Product Design link to old md page
[#79] Update link to thoughtbot style guide
Add canonical design trail

Start to add main design trails and edit others
Combine html and css trails
Polish up design and web design maps
Add Reda's fixes and user experience
Add requirements for webdesign trail
Revisit visual principles
Use real jsonlint link not link to its repo
correct visual principles reference
correct for css & html combination
fix rails html-css prerequisite (again)
Remove rails.md
Add the RubyMotion book from PragProg
Always refer to current Postgres docs