Add namespace

[rekire]

Fixes #108

Please note that I forked in the past a fork. Therefore the change is a bit bigger. However I see no problem when those changes go online too.

For others who might want to test it:

dependencies:
  // other dependencies
  flutter_libserialport: #^0.5.0
    # remove this git part when 0.5.0 is published
    git:
      url: https://github.com/snabble/flutter_libserialport.git
      ref: 0.5.0

[ElHyperion]

Why is this still not merged?

[arnovanliere]

@jpnurmi can this be merged and a new version released? This blocks me from updating my app.

[LuvForAirplanes]

This is holding up the show, please merge.