Allow newlines and other control characters in JSON string literals.

This is to allow multi-line base64 encoded data in the "contents" attribute of img_personality. (There's no particular reason to allow newlines in img_properties but I did this for consistency)

Fixes #51

Tested by loopback-mounting `/var/lib/snf-image/helper/image` and manually modifying

* /usr/lib/snf-image-helper/decode-properties.py
* /usr/lib/snf-image-helper/inject-files.py