[tkt-10] Saves thumbnails in subfolders

Added Image/Finder class for image file name/folder manipulations.
It provides methods for getting unique path for a thumbnail image using
first three characters from md5 hash as a folder name and the rest as
a file name.

All Commands are now using it to get all kinds of thumbnails files or
folders.

Also refactored GD Image class and added factory method.
Merge pull request #1 from hoborglabs/tkt-10

[tkt-10] Adds ImageFinder class
[tkt-54] Adds support for '--config' option

You can specify where is your configuration file using --config or -c.
Default value is 'sgallery.properties'