Show compressed text messages in wsdump.py

Since https://github.com/websocket-client/websocket-client/pull/506
changes the way that options['header'] is used, it breaks --headers
option of wsdump command because wsdump command uses a map object
instead of a list here, which can only be used once. Therefore, after
the first use with 'not in' operator, it becomes empty and no headers
specified by the user are sent.

To fix it, simply convert the map object into a list, so it can be use
multiple times.

Previously, because of the multiple instances of `from foo import *`, names were
imported from modules that had themselves imported them, instead of from the
place of definition. This commit therefore does the following:

- Declares `__all__` in every module that imports anything, so that
`from foo import *` is sane.
- Sorts the imports based on conventions, similar to the output of `isort`.
- Places all conditional imports after unconditional imports, except where that
isn't valid.
- Imports local names from the modules where they are defined, except when
importing the package itself.