20103 responsive web should handle the situation where ibl is unable to return an image

https://jira.dev.bbc.co.uk/browse/IPLAYER-20103

Associated Bamboo PR found at:
https://github.com/iplayer/responsive-web/pull/3046

[ORIGINALLY SUBMITTED 02/10/2014]
- Updated `getImage` within the `Elements` model to return a URL to the placeholder image (uploaded in separate PR)
- Updated the `ElementsTest` accordingly

[UPDATED 06/10/2014 @ 15:10]
- Created new static `Configuration` class, which is used to hold values specific to the client that are used by Bamboo objects; transferred all relevant members of the `Client` class to this class and associated unit tests were rewritten within `ConfigurationTest.php`.
- Added `$_placeholderImageUrl` member variable to the `Configuration` class, which is used to define the image that is used when iBL is unable to return an image.
- Updated `Elements.getImage()` and `Elements.getStandardImageRecipe()` to make use of `getPlaceholderImageUrl()` within `Configuration` class

[UPDATED 07/10/2014 @ 10:42]
- Renamed `$l` in `ConfigurationTest.testSetCache` to `$clientListeners`

[UPDATED 09/10/2014 @ 14:24]
- Moved duplicated code in `request` and `requestAll` to private method named `_getRequestObject`
Put in variable at the top?