style(rxForm): Form field layout changed to use flexbox

BREAKING CHANGE: While no visual style changes are intended, due to the
nature of switching from floats to flexbox, it's likely that the visual
style of form fields will be altered, especially if custom css has been
added to the app.
fix(rxForm): add dynamic label field id integration
docs(rxForm): add details on `label` script
Merge pull request #370 from rackerlabs/355-form-flex

Switched form layout to use flexbox