Add appveyor configuration

[antograssiot]

• correct the composer autoloading lines generation on Windows
  using

 bin\cake bake plugin BestPlugin

would generate the following in composer.json :

"BestPlugin\\": ".\\plugins\\BestPlugin/src"

instead of

"BestPlugin\\": "./plugins/BestPlugin/src"

• re-add missing functions to bake Plugins after RFC: Remove the project task #43