Clear cache when new webviews are instantiated.

This might have some disadvantages, but the advantage is that
changes to your included javascript files get loaded. Else you're
stuck with the first version you push/load for a while.
(cherry picked from commit 99f47a28cce19fcc988c22dd72dc617176347009)
Merge pull request #68 from swgreen/dev-fresh1

Clear cache when new webviews are instantiated.