rewrite asset paths inside srcset lists

[chadkirby]

Fixes #51

According to MDN, the srcset attribute contains:

A list of one or more strings separated by commas indicating a set of possible image sources for the user agent to use. Each string is composed of:

1. a URL to an image,
2. optionally, whitespace followed by one of:

• a width descriptor, or a positive integer directly followed by 'w'.
• a pixel density descriptor, which is a positive floating point number directly followed by 'x'.

The rewriteAssetPath logic seems to behave properly if we simply allow assetPaths to begin/end with commas (in addition to quotes, open paren, and equals). The match will include the width/pixel-density descriptor at the end, but that does not appear to cause problems (in the admittedly few cases that I've tested).

[rickharrison]

This looks good to me. Anyone from the Ember team want to confirm everything looks good inside a real world app?