Don't clear background images and icons on WP8 tiles by default

If an app sets the icon/image locally, then omitting a value and setting
clear will cause the icon/image to disappear. Leaving this attribute off
preserves the existing icon/image.

This does complicate things, since some elements will clear automatically,
and now others won't. It may make sense to denote a special value that causes
the clear to be set, or provide a richer interface.

Also updates version (1.0.0 -> 1.1.0).
Peg webmock to 1.9.0

HEAD on github is failing, so let's stick a known good version.