Allow the indexedDB implementation to be configured.

This enables a workaround for the unshimmable window.indexedDB in iOS 8.0's
UIWebViews.

References:

- https://github.com/axemclion/IndexedDBShim/issues/161
- https://bugs.webkit.org/show_bug.cgi?id=137034
Merge branch 'use_other_indexeddb' into dist
Add the changes from the use_other_indexeddb branch.