Switch to tilde version range for source-map-resolve

It is 0.x and may introduce backwards-incompatible changes in minor
version increments.
Merge pull request #31 from lydell/master

Switch to tilde version range for source-map-resolve
Remove the `position` option of `css.parse`

It was enabled by default, makes errors and source maps more helpful,
and there’s no reason to ever turn it off, so it’s better to simply
remove it.
Merge pull request #33 from lydell/no-position

Remove the `position` option of `css.parse`
Fix #11: Update example parse tree
Fix #21: Document the AST
Improve readability and consistency in readme

By reducing the amount of `code` tags.
Minor style nits
Merge pull request #35 from lydell/document-ast

Fix #21: Document the AST
Add non-enumerable parent reference to each node. Fixes #19