20103 - Handle the situation where iBL is unable to return an image

- 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