Upload VCFs

Here's what the interaction looks like:

![screen recording 2014-12-04 at 06 54 pm](https://cloud.githubusercontent.com/assets/98301/5308631/121c19f6-7be7-11e4-8d6b-b720fced8d61.gif)

This uploads the file to a known directory on the server and fills in a `file://` URL (which this PR also adds support for).

Fixes #217
Fixes #324


I'm aping Gmail, I'm not sure if it's a convention:

![drop target](https://cloud.githubusercontent.com/assets/98301/5318027/8332652c-7c6a-11e4-9a28-409501a69a20.png)