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
Fix 'invalid thread id' issue #13 by making sure tid is a C long int.
Changed WindowsErrors to OSErrors to support non-windows platforms
Moved tests outside of the actual module and changed setup file accordingly
Changed tests from UnitTest to py.test
A few more tests for packet operations
Added pretty print function to packet/layer (to print with colors)