Copy password from password list

Hi!

A simple PR you may consider.

By using Rattic on a daily basis, it appears the user flow to get a password is too slow for us. It indeed requires the user 3 actions:

1. Click to open the password detail page, wait for its loading.
2. Mouse over the scrambled password text to make the *copy* button appears.
3. Click the *copy* button.

This PR adds a *copy* button directly in the password list, so copying the password is only one click away. 

For us it makes the use of Rattic as fast as KeePass from where we come from (ctrl+c on the password or *right click > copy*).

This new *copy* button works the same than the *copy* button on the password detail page and thus does not break others feature like auditing for instance.

![image](https://cloud.githubusercontent.com/assets/1324566/4868010/0a98ded8-613b-11e4-9914-ae24b215ea50.png)