pat-bumper: rewrite to handle pinning in scrolling containers

This fixes #369
Merge branch 'master' into bumper-in-scrollable
Use jQuery CSS to get current styling

Using <element>.style only returns element-local style settings, and we
must use the full computed style data.
Name an unnamed function.
Bumper: use computed overflow-y style
Bumper: only chance container position when needed

There is no need to set the containers position to fixed if it already
is the offset parent.
Bumper: always use possibly scrolling parent as container

This fixes problems with a container not being scrollable right now, but
becoming scrolling when something else change (for example window
resize).
Fix tests after last change.
Bumper: preserve old style on init failure.
Add changelog entry for bumper changes.