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
Merge pull request #86 from gaurish/master

Always link to docs of current postgres version
Add script for validating JSON

* Add classes to validate JSON
* Add Gemfile for supporting libraries
* Add script to contribution guidelines
* Split out classes into separate files
* Require gems in main TrailRunner class
* Expand error message to recommend jsonlint.com
* Write specs

Other changes:
* Move script into `lib/tasks/` and make directory-agnostic
* Sort gems alphabetically
* Don't require mocha
* Add awesome_print to Gemfile for debugging
* Clean up Gemfile
* Specify ruby version in Gemfile
* Sort gems
* Remove awesome_print (used only for diagnostics)
* Change mocha require to remove deprecation message
Update trails/ruby.json

Fix typo in Ruby trail map
Convert Product Design to json

Removed the 'Hold the Mayo" from the validations since the link format
doesn't match the rest of the trails. It was included inside the Getting
Real resource anyway, so it doesn't really seem needed.
[#85] Update link to Ruby Core API

Update Ruby trail to link to current Ruby core documentation