add default template option to Package()

* add acceptance test: 03-change placeholder text
* add pptx.presentation.Package constructor called with no parameters
  loads default template
* refactor pptx.presentation.Package().open(path) to Package(path=None)
* add acceptance test: 04-default-template.feature
* test Package.save(path)
* add save() method to Package
* add hello world image and example to docs homepage