index: lowercase callback function name

Makes it consistent throughout the file
README++
index: pass the `sandbox` option in to PacResolver
index: calculate an SHA1 hash of the JS code

Allows us to re-use a previous resolver instance if the code has not
changed since last time (but ENOTMODIFIED was not returned).

This is common for when a remote HTTP script returns dynamic content,
and therefore can't set caching headers, but it's usually the same result.