Improve searching and image matching capabilities

# Improved search for images
* More than one directory may be searched
  * List of directories is now expected in the form of positional command line arguments
  * The -p option previously used to select the target directory is still supported
  * If no directories are provided (no positional command line arguments and no usage of -p) then the current directory is used
* Directories can now be searched recursively (using -R)
  * Symlinks can be followed when searching recursively (using -f)
* Image formats are now detected using the python `mimetypes` modules
  * It extension based and therefor not always reliable but its a major improvement nonetheless

# Minor clean up
* Disabled --set-background option (since it doesn't work outside of GNOME 2.x)

# Updated README
* Made text in README mention the fact that there are several desktop environment supporting this format nowadays
* Updated the command line help of `gnome-background-generator`
* Added two examples for usage of `gnome-background-generator`