Check presence of required external tools for each comparison engine

[dgsuarez]

The core of Similatron are comparison engines. Each engine works on different file types, and most of them use external tools, for example Imagemagick (for image files) or Ghostscript + Imagemagick (for PDFs). If a tool is missing that engine should not be used.

It could be as simple as having a class method in each of the engines to check if the executable exists, and then in Run#build_engines select only those engines that are available.