BitmapPreview class to handle all Imagick conversions to bitmap

As I was considering more formats to play with, I quickly realised that the process was always the same. Imagick does a good job out of the box to properly convert files to bitmaps.

I'm always targeting the layer 0 of the file to convert as it's the one which either contains the first page, the only image or a flattened representation of a document.

Bitmap is in the name of the class because I can see a separate class being created for people who have an improved version of Imagemagick and who want to generate SVGs

And finally, .psd, .ai and .eps support have been added.
As explained in the OP. There might be another class for SVG output using Imagick, so I was trying to avoid confusion.