Stay below shebang line

[schnittchen]

The plugin inserts the encoding stanza at the top, regardless of whether there exists a shebang line

#!/bin/ruby

or similar, which needs to be on the first line (encoding stanza just below works at least in ruby 1.9.3).

[schnittchen]

I did some work on this problem here https://github.com/schnittchen/auto-encoding-for-ruby/tree/respect-shebang

Before creating a pull request, I'd like someone to test it. I would also like to change to 4-spaces indenting to reduce linter noise, and do a little cleanup.