Resolve rikulo#163 Null safety support for 2.0.0-beta

Fix rikulo#150 Problem with setQuery in socket io version 3.0

Fix rikulo#140 getting Error on emitWithAck() in v2 beta

fine tune document for 2.0.0-beta.0

1. Resolved rikulo#130 Cannot connect to socket.io V3

2. Resolve rikulo#106 Can we combine emitWithBinary to emit?

Resolve rikulo#132 Migrating to null safety for Dart

Resolve rikulo#46 Make this library more "Darty"

Resolve rikulo#108 Need dispose method for clearing resources

Fix dart analyzer warning for formatting issues.

Fix dart analyzer warning