formatting Hash as comma separated key=value

Currently Hashes are formatted in the log message like this:

    2013-09-19T10:35:25-07:00 [ INFO] 8980 :  simple_string: simple, string_with_spaces: two and three

Many log analyzers prefer key=value pairs though. Here is a humble pull request to change the Hash formatting to this more parseable format. For example,

```ruby
    logger = Yell.new STDOUT
    logger.info simple_string: "simple", string_with_spaces: "two and three", number_var: 234
```
would result in

    2013-09-19T10:35:25-07:00 [ INFO] 8980 : simple_string=simple, string_with_spaces="two and three", number_var=234

If this is a too radical general change - in case you consider this at all helpful - I could imagine a new 'message' formatter variable instead.