Add code quality checking via phpcs, eslint, jscs, and jshint

[westonruter]

• Limit checking of code to the lines that are modified in scope of the pull request; this will allow for adherence to linting rules to be applied incrementally in the context of PRs without having to fix everything up front in order to get a passing build.
• Fix i18n errors related to UnorderedPlaceholdersText and MissingArgDomain.
• Add wp-dev-lib, which is added as submodule.
• Re-use phpunit bootstrap from wp-dev-lib.
• Update Travis to use container-based environment and use caching to improve performance.

[amedina]

Adding wp-dev-lib into the development workflow of the plugin is great! Spent some time looking into wp-dev-lib and the changes in this PR look good to me.

[amedina]

Ready to merge.