Add #single_line_exceptions feature.

This adds a new feature for managing exception data in generated log messages. Default behavior is to generate a log message for each line of the backtrace, the result of which could be many lines for a single exception. To cut down on duplication and data transport this generates a single log message for the exception, joining all backtrace lines by `'\n'`.

### Before

```
now="2014-01-23T13:22:32Z" app=scrolls deploy=nil at=exception class=NoMethodError message="undefined method `empty?' for nil:NilClass" exception_id=70161806013660
now="2014-01-23T13:22:32Z" app=scrolls deploy=nil at=exception class="undefined method `empty?' for nil:NilClass" exception_id=70161806013660 site="./test.rb:16:in <main>"
```

### After

```
now="2014-01-23T13:22:59Z" app=scrolls deploy=nil at=exception class=NoMethodError message="undefined method `empty?' for nil:NilClass" exception_id=70116515914640
```