Addition of attrs property to image template tag
Merge branch 'imagetags-attrs-prop' of https://github.com/jeffrey-hearn/wagtail into jeffrey-hearn-imagetags-attrs-prop
use self.attrs in AbstractRendition.img_tag to avoid duplication
Document the 'attrs' property of image renditions
Changelog entry for #299
Merge branch 'jeffrey-hearn-imagetags-attrs-prop'