Extract: rename method `extract_entries` to `extract_from_code`

The other extract methods are named `extract_from_<something>`, this was
te only one, which wasn't.

Also, it was returning an instance of `Translations`,
not an array of entries, which the name suggested.


git-svn-id: https://develop.svn.wordpress.org/trunk@25920 602fd350-edb4-49c9-b593-d223f7449a82