Import parsing replacement

Completely optional pull-request. Feel free to ignore it. 

I replaced "parse-import" with "postcss-helpers" and updated places where it is called to make working with import rules more straightforward:

  1. RegExp in `postcss-helpers` is more in line with http://www.w3.org/TR/CSS21/grammar.html and returns more clean media queries (for example it removes \n symbols from beginning and the end of the string, than they are used as separator). See https://github.com/iAdramelk/postcss-helpers/blob/master/lib/regexp.js vs https://github.com/kevva/parse-import/blob/master/index.js
  2. You don't need to add `@import` to parse it anymore.
  3. Replaced `parsedAtImport.path.match(/[a-z]+:\/\//i)` with parsedAtImport.URI.is("absolute") to also detect urls that are starting with `//`.
  4. Replaced `parsedAtImport.path` to `parsedAtImport.URI.path()` because it returns path without `#` and `?` segments that can be used to set version numbers.