fixed inconsistance with prime numbers

[arthurvergacas]

• Improved performance in the prime generation

• Changed way that message is displayed to user

• Removed mod function

• A problem was occurring with prime generation: low or equal p and q resulted in wrong decryption.
• Also improved the prime generation performance: now, it only test odd numbers
• The mod function wasn't necessary, and could be replaced with the modulo operator (%)
• Lastly, simply joined the characters in the final list so the user could see more clearer that the decryption was successful

Also, formated the code properly, so it's easier to read ;)