Add favicon

When documentation is generated using the template 'clean', and
it is viewed using a browser, no favicon is displayed in the
browser tab. Fix this by copying favicon.ico and
apple-touch-icon-*.png from the img directory of template
'responsive'.

Fixes phpDocumentor/template.clean#18
Merge pull request #19 from susam/fix-phpDocumentor/template.clean#18-favicon

Add favicon