Improve solution for #147

Now it won't clear all other settings in the viewport meta.

It changes `width` to `320.1` if its value is `320`, or removes it if its value is `device-width`.
Reinstate `initial-scale=1` in meta viewport defaults (close #140)
Merge pull request #151 from oscar-broman/patch-2

Improve solution for #147 (iPhone 5 startup image)
Update CHANGELOG.md