Check for S3 write permission and show warning if can't write

Resolves #124 

Checking the permissions of a user doesn't seem easy within the AWS SDK ([related](https://github.com/aws/aws-sdk-php/issues/121)), so went with this implementation.
Without those styles it looks like - 
![screen shot 2014-10-21 at 12 49 30](https://cloud.githubusercontent.com/assets/1770201/4717723/5e6f4504-5918-11e4-9e86-7a78be696c6b.png)