Add single-byte XOR cipher

[ghost]

Resolves Issue #150

Description

Created systems/150 folder and added systems/150/single_byte_xor.py which contains an implementation of single-byte XOR cipher.

Checklist

• [x] Code compiles correctly.
• [x] Changes are tested properly.
• [x] Followed the Contributing guidelines.
• [x] Resolved merge conflicts, if any.

[ryan-rozario]

Looks good @SaurabhAgarwala Please merge PR