Added the ability to store a sheet at any moment (abstracted thanks to
SheetStorageInterface).
Added the ability to read a sheet from a file, through brand new Sheet
managers. The file from which the sheet is extracted can be chosen by
the user or by a random selection according to level of difficulty.
Added the ability to store a sheet at any moment (abstracted thanks to
SheetStorageInterface). Storing in files is available.
Added the ability to extract a sheet from a file, which would have been
chosen by the user, or according to the required difficulty level.

TODO: comments and javadoc