RE: ISR is never called

I suspect most of your problems stem from a mistake in your initialisation:
Initialize
        ;-------CHANGING TO BANK 1 ----------
        BSF     STATUS, RP0     ; Select Bank 1

        ;*****  SET UP PORTS AS INPUTS OR OUTPUTS  *****
        CLRF    PORTA           ; Initialize PORTA by setting output data latches

Incorrect!!  You are in bank 1 and cannot access PORTA from here.

        MOVLW   0000h           ; Value used to initialize data direction
        MOVWF   TRISA           ; ALL 5-pins set as output

This is fine.

        CLRF    PORTB           ; Initialize PORTB by setting output data latches

Again, incorrect.  PORTB is in bank 0.

        MOVLW   0001h           ; Value used to initialize data direction
        MOVWF   TRISB           ; ALL 8-pins set as input

All 8 pins are not set as inputs.  You have only set pin 0 as input, the rest as outputs.

        ;*****  ASSIGN TO TMR0  ****
        bcf     OPTION_REG,PSA  ; PSA = 0
        ;Set Prescaler to 16    [011=PS2|PS1|PS0]
        bsf     OPTION_REG, PS0
        bsf     OPTION_REG, PS1
        bcf     OPTION_REG, PS2

        ;-------CHANGING TO BANK 0 (default) ----------
        BCF     STATUS, RP0     ; Select Bank 0


As you are using a 16F84 you might like to consider using the TRIS instruction to save having to set banks.

e.g.

        movlw   00h
        tris    PORTA           ; set all PORTA as all outputs
        movlw   FFh
        tris    PORTB           ; set all PORTB as all inputs
        clrf    PORTA
        clrf    PORTB

Hope this helps

Mike Rigby-Jones