Fixed margin collapse bug when using `normalize()` and the Vertical Rhythm module together

When using the Vertical Rhythm module added in 20eff22683b4c9ea571d1e060d4e84af9ff51c1b, and using the `normalize()` mixin, some very horrible collapsing margins appear at the top of the window pushing all content inside `<body>` down by about 30 pixels.

This pull request fixes that by making the `normalize()` mixin aware of the baseline grid. `normalize()` now accepts an optional `boolean` parameter: `baseline-support` that defaults to `false`.

In order to trigger the new behaviour, simply pass `true` to `normalize()`, and it'll do all the math for you.

A necessary change was to `@import 'vendor'` before importing the reset, so that it could make use of the `box-sizing` vendor mixin.