The modulo method now works with negative numbers.

Also added a egcd method to get the greatest common divisor of a
and b by using the euclidean division.

Also added comments to all methods.

The create_subkeys_table method now directly create a 2d array
containing the subkeys for each rounds.

The create_decryption_subkeys_table and encrypt table have been
updated in accordingly.

Small refactor global of the code to add global constants.

Added an optionnal parameter to the shift_key methods to determine
the number of bits to shift.

In order to decrypt the ciphertext, we must :
- be capable to calculate the modular inverse
- create the decryption subkeys table
    - this required others methods to create a 2d table of the
    encryption subkeys

The code of the method is cleaner and easier to read.

Improved the add_mod and mul_mod methods to supports
string parameters.
Fixed an error in the mul_mod method that wasn't
converting 0000 nibbles as 16 (base 10).