Add image to Account

This adds an image URL to the Account model.

This is useful for capturing a logo for an organization.
![new_field](https://f.cloud.github.com/assets/365751/1234369/88148958-296e-11e3-8f4d-67a7f58cabd8.png)

It displays like a gravatar would in the sidebar
![screenshot from 2013-09-30 10 39 08](https://f.cloud.github.com/assets/365751/1234368/8799cd12-296e-11e3-8570-807e4a3d6355.png)

@mikecanham has added validations, migrations, etc

It's part of work we are doing to link linkedin company information to fat free crm records (ie: consuming http://developer.linkedin.com/documents/company-lookup-api-and-fields); happy to open that up if the community would be interested.