Fix inaccurate docstring
XML detection fails in certain character encodings

pyshark attempts to detect XML files, but this will fail for PDML files
that don't match ANSI encoding
Merge pull request #28 from jeffito/master

Fix 'invalid thread id' issue #13 by making sure tid is a C long int.
Merge pull request #27 from bbayles/minor_filecap

Minor file_capture fixes