Token plugins provide a generic interface for transforming a token
into a `(host, port)` tuple.

The plugin name is specified using the '--token-plugin' option,
and may either be the name of a class from `websockify.token_plugins`,
or a fully qualified python path to the token plugin class (see below).

An optional plugin parameter can be specified using the '--token-source'
option (a value of `None` will be used if no '--token-source' option is
passed).

Token plugins should inherit from `websockify.token_plugins.BasePlugin`,
and should implement the `lookup(token)` method.  The value of the
'--token-source' option is available as `self.source`.

Several plugins are included by default.  The `ReadOnlyTokenFile`
and `TokenFile` plugins implement functionality from '--target-config'
(with the former only reading the file(s) once, and the latter reading
them every time).  The 'BaseTokenAPI' plugin fetches the value from
an API, returning the result of `process_result(response_object)`.
By default, `process_result` simply returns the text of the response,
but may be overriden.  The `JSONTokenAPI` does just this, returning
the 'host' and 'port' values from the response JSON object.

The old '--target-config' option is now deprecated, and maps to the
`TokenFile` plugin under the hood.

Also-Authored-By: James Portman (@james-portman)

Closes #157

Don't use implicit relative imports

Implicit relative imports don't work in Python 3.  This converts
implicit relative imports into absolute imports
(e.g. `import websocket` becomes `from websockify import websocket`).

Fixes #154

Added 'websockify.js' to package.json files list

python3 compatibility fixes

Fix typo in reaping zombies message.

Added websockify.js to the files section, this fixes #90 and allows NPM to properly install