Add inline documentation

Closes #226
Put param description on a new line
Remove placeholder parameter in docs
Add unit info in param and variable type in docs
Add inline docs for new-breakpoint
Replace en-dash with regular dash
Improve docs readability
Refactor private functions

    - Closes #243
    - Use null instead of nil (closes #234)
    - Ignore case for string variables (closes #235)
    - Remove extraneous @else (closes #237)
    - Use is-even where appropriate (closes #238)
    - Remove ternaries (closes #239)
    - Remove camel case (closes #240)
    - Refactor is-display-table (closes #241)
Rename direction and display mixins

- Prevents a conflict with Bourbon's `display()` mixin. Closes #231, #228.
Bump to 1.7.0.rc
Update logo and docs in README
Switch to inline style comments (///) for docs
Update border-box-sizing setting example
Fix libsass compatiblity

- Inverting the order of `!default` and `!global` seems to do the job for now.
Add travis
Use full import path for convenience

  - This makes it easier to test locally with libsass
  - Closes #253
Bump to 1.7.0
Refine README, LICENSE and CONTRIBUTING docs