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.

It is now literally this easy to have sensible defaults that conform to a vertical rhythm:

```
@import 'axis'

base-font-size = 16px
base-line-height = 24px
normalize(true)
```

If you want proof:
```
body
  debug-vertical-alignment()
```

![picture](http://gyazo.com/b5e77152b52324ae6bd43ebf7f42fb6b.png)

Change either `base-font-size` or `base-line-height` to any value and it should still conform to vertical rhythm.