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.