Release "unified execution" for scripts and styles.

Promote previous `experimental-execute-unified` logic to be always enabled.

Inline script and style evaluation now always uses two functions:
- `eval`: conditionally evaluate the text only if changed
- `exec`: unconditionally execute the text

This matches the behavior of external script and style installation:
- `load`: conditionally install the resource only if changed
- `get`: unconditionally install the resource.

When processing a SPF response, the script and style execution rules are now:
* If an inline `<style>`, inline `<script>`, external `<link>`, or
  external `<script src>` with a `name` attribute is parsed, only execute
  if a style/script has not already been executed that matches the URL or
  text content.  Remove all other scripts and styles with the same name after
  executing.
* For all other scripts and styles (without a `name` attribute), execute
  unconditionally.

Progress on #25.
Merge pull request #131 from nicksay/unified-execution

Release "unified execution" for scripts and styles.