Giving a non-JS file should throw an error

Closes #67
Fixed release notes link
Errors in the callback of webpage.open() are now displayed
webpage.clipRect should not be null by default
Fixes screenshot size of webpage.render()

It generates now screenshot of the full webpage, instead of the viewport area
More improvements on webpage.render()

- scrolling is taken account for screenshot
- scrollbars have been removed
- new onlyViewport option
Merge branch 'v0.8'

Conflicts:
	docs/release-notes.rst