Refactored _dec8 as _decR
Refactored _ld8 _ldRn
Added generic 'ADD HL, RR' method
Added generic 'LD A, (RR)' method
Refactored _dec16 as _decRR
Added 'LDI A, (HL)'
Added 'DEC HL'
Added 'INC L'
Added 'DEC L'
Added 'LD L, n'
Added 'CPL'
Renamed test_CPL() to test_cpl()
Added 'JR NC, n'
Added 'LD SP, nn'
Added 'LDD (HL), A'
Added 'INC SP'
Added 'INC (HL)'
Added 'DEC (HL)'
Added 'LD (HL), n'
Added method for getting value of HL.