Utilities: added the 'getFileExtension' function.

This function returns the extension of a file as a std::string.
Image class: filename parameters are now passed as constant.
main: Added automatic output format definition.

The program analyzes the file extension and saves the image according to
the given format.
Added a default output file and format.

If no output file and/or format are specified, the camera will save the
image as a PNG file called "image.png" in the working directory.
main: Added the '-f' command line option.

This option takes an argument and specifies the output format. Using
this option disables automatic extension checking. It may be used to
use arbitrary file names that do not have the usual extension.
Merge branch 'automatic_file_format'