v1.4.0: $uptime device property and ability to subscribe to all properties of a node

Thanks to everyone involved.

New features

• Implement the $uptime device property, refreshed by default every two minutes (#52)
• The ability to subscribe a node to every properties. This is useful for a led strip for example: you will not want to do node.subscribe("1"); node.subscribe("2"); ... if you have a thousand LED. So now, with a fourth parameter to the HomieNode constructor, you can subscribe to everything. See the LedStrip example for a concrete use case (#62)

Fixes

• Fix an issue where reconnecting to the MQTT with SSL enabled would crash (#54)

Web UI

• The default hardware device ID was not used if the device ID was not set in the Web UI. This is fixed (#63)

Misc

• The AP IP is now configurable from the Constants.hpp file (#56)
• Callbacks are now std::function - thanks to @rschaten (#59)
• More dynamic allocations avoided when logging