frame.create fails if stylesheet or html specified does not exist
INTEGRATE: 0.3 documentation and QElement.assert()

* dev:
  Readme tweaks
  API documentation updates for 0.3
  Example is updated for v0.3
  QElement.assert()
  Changelog updates for 0.3
  Roadmap updates for 0.3
Merge branch 'createFrameWithMissingFile' of https://github.com/bjornicus/quixote into dev
Code style tweaks
Contribution credit for @bjornicus
Frame.create() enhancement in changelog
INTEGRATE: Frame.create() fails fast when URL not found

* dev:
  Frame.create() enhancement in changelog
  Contribution credit for @bjornicus
  Code style tweaks
  frame.create fails if stylesheet or html specified does not exist
0.3.0