Further ios file previewing (and now, sharing)

File viewing and sharing etc courtesy of `UIDocumentInteractionController` and `UIActivityViewController`.

Requires the [iOS branch](https://github.com/SpiderOak/FileViewerPlugin/tree/iOS) of the `FileViewerPlugin`.

![screen shot 2013-11-01 at 4 12 51 pm](https://f.cloud.github.com/assets/554999/1461510/7820adf2-44b6-11e3-8a22-d2d1321844f9.png)