Grouping coverage report by service

Small tweak on the coverage config, makes it easier to spot where test coverage is needed the most.

Quick recipe to test it:

```
git clone git://github.com/rafaelrosafu/aws-sdk-ruby.git
cd aws-sdk-ruby
bundle install --path=vendor --binstubs
COVERAGE=true bin/rake spec
```
open  `coverage/index.html`

I added `bin` to `.gitignore` to avoid issues with `binstubs`.

Cheers