Setting OSX defaults with a key with spaces requires it to be quoted.

Hi,

I was unable to set OSX defaults for things like iCal, as some of the keys have quotes.

As per example:
![Screen Shot 2013-04-02 at 11 19 17 AM](https://f.cloud.github.com/assets/19973/326298/d8eca5da-9b2b-11e2-93bf-d4e6d72aba83.png)

Attached code ensures the key value is quoted.

My current workaround to get this to work with boxen (before this pull request is merged) is to include in the key, which is a bit fugly:

```
boxen::osx_defaults::tommeier { "TimeZone support enabled":
    domain => 'com.apple.iCal',
    key    => "'TimeZone support enabled'",
    type   => boolean,
    value  => true
  }
```