Add relative formatting support and locale data

This refactor brings `dust-intl` inline with the other integration
packages: `handlebars-intl` and `react-intl`. This package now supports
relative-time formatting and also includes all the locale data for the
150+ supported locales (English is baked in as the default.)

_Note: The changes to `dist/` are left out of this PR so it's
reviewable._
Update intl-messageformat and intl-relativeformat
Remove trailing comma in Gruntfile
Merge pull request #41 from ericf/relativeformat

Add relative formatting support and locale data