Add attachments support for messages #467

I took a simple approach including a multiple file input into the message form, no async upload nor Javascript plugin.

Rails 4.1.0 [have issues with file upload and strong parameters](https://github.com/rails/rails/issues/9051), so I updated it to 4.1.1

To keep the form style I hid the file input and added a button to interact with user. This button triggers the file input click event, and when the file input is changed the button text changes to show how many files the user is attaching in the message.