[tkt-54] Adds support for '--config' option

You can specify where is your configuration file using --config or -c.
Default value is 'sgallery.properties'
Merge pull request #2 from hoborglabs/tkt-54

[tkt-54] Adds support for '--config' option
[tkt-58] Unifies folder and file creation mode

Image/Finder is now used to create missing folders and ensure that
correct access rights are set on files.
This commit also changes the way commands objects get their
dependencies. Each object implements 'inject' method that can finally
by used to inject mocks in tests.