Handle more optional modules for OpenWrt

[timothyqiu]

A major issue is that the json module is broken for python-light on OpenWrt. The module won't work without another system package called python-codecs.

As an alternative Python package, python-base on OpenWrt provides a much smaller subset of the Python standard library than python-light. Modules like atexit, tarfile, time, binascii, and even __future__ are all unavailable.

These modules are all imported without checking their existence.

[fffonion]

__future__ module can't be wrapped in a try statement. It's better to add proper notice in Readme then change the code.