Fix RTD badge

Merge pull request nodemcu#883 from nodemcu/dev

Resync master to dev prior to upgrading Dev to SDK 1.5

Fix documents.

Add node.restore() to restore Wi-Fi.
Changed wifi.startsmart, see also https://github.com/nodemcu/nodemcu-firmware/wiki/nodemcu_api_en#wifistartsmart.

fixed ap/station-ap cannot connect to the device.

added wifi.ap.getconfig().
fixed net.dns.getdnsserver().
added new base64 lua example.
added node.bootreason() to inspect boot cause.
optimization of u8g.

Johny Mattsson's commit almost doubling the available Lua heap.

bump to 0.9.6

update mqtt, see readme.md

quick fix cjson.encode

add cjson module