Minor file_capture fixes

Here are two small fixes for minor issues with `file_capture.py`.

First - the docstring for `packets_from_file` is outdated; it doesn't return a tuple.

Second - XML detection fails for files with certain encodings, particularly those with a byte order mark (I can attach an example, but basically [the XML spec](http://www.w3.org/TR/xml/#sec-guessing) describes a slightly better way to do the detection for common encodings).