[WIP] Port to ES2016, Refactor & Enhancements

[zspecza]

closes #31, #35, #39, #47, #49, #51, #53

• add notes to docs regarding debugging requests
• update code to ES2016
• remove runtime dependency on lodash & when
• swap out string.js for underscore.string
• use Standard JS Style
• add lint script
• add pre-commit linting
• remove risky unbuild step
• upgrade tests
  • port current test suite to AVA
  • use nyc for coverage
  • improve test performance
    • run tests concurrently
    • add async: true option to roots-util test helpers (Test Helper Changes roots-util#7)
    • update roots-contentful tests to take advantage of the new async options in roots-util
  • improve test stability
    • disable analytics in roots-util helpers (carrot/roots-util@7935ae8)
    • replace deprecated fs.exists methods with fs.stat in roots-util test helpers (carrot/roots-util@ef62034)
  • reduce boilerplate
  • ensure separate test files don't modify the same fixture
  • assert error messages, not just the fact that errors are thrown
  • add missing test for asset_view_helper else path
  • add test case alongside roots-yaml and roots-records
    • roots-yaml
      • roots-contentful data in roots-yaml
      • roots-yaml data in roots-contentful
    • roots-records
      • roots-contentful data in roots-records
      • roots-records data in roots-contentful
• new features
  • add locales support from add locale(s) support #39
    • internal logic
    • tests
    • needs refactoring - see [WIP] ES2016 locales #64 for progress
  • add caching
    • internal logic
    • tests
  • add _path helper for single entry views
    • internal logic
    • tests
  • enable use of _url and _urls locals inside content types that reference each other
    • internal logic
    • tests
• ensure documentation and contribution guidelines reflect the new changes

note to self: document that this is a breaking change which requires npm 3 and node 5

[hhsnopek]

we should release this as v1.0.0 once stable :)