Side buttons for mouse -- how to enable -- logs included

I have a mouse with two side buttons.

I've posted the output from xev and evtest for both buttons:

xev

EnterNotify event, serial 37, synthetic NO, window 0x4600001,
root 0x306, subw 0x0, time 5663259, (11,69), root:(799,121),
mode NotifyNormal, detail NotifyInferior, same_screen YES,
focus YES, state 0

KeymapNotify event, serial 37, synthetic NO, window 0x0,
    keys:  6   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   
           0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   

---------------------------------------------------------------------------
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EnterNotify event, serial 37, synthetic NO, window 0x4600001,
    root 0x306, subw 0x4600002, time 5662311, (30,60), root:(818,112),
    mode NotifyUngrab, detail NotifyVirtual, same_screen YES,
    focus YES, state 0

KeymapNotify event, serial 37, synthetic NO, window 0x0,
    keys:  6   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   
           0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   

evtest:

Event: time 1420063746.029705, type 4 (EV_MSC), code 4 (MSC_SCAN), value 90005
Event: time 1420063746.029705, type 1 (EV_KEY), code 276 (BTN_EXTRA), value 1
Event: time 1420063746.029705, -------------- SYN_REPORT ------------
Event: time 1420063746.165669, type 4 (EV_MSC), code 4 (MSC_SCAN), value 90005
Event: time 1420063746.165669, type 1 (EV_KEY), code 276 (BTN_EXTRA), value 0

------------------------------------------------------------------------------
------------------------------------------------------------------------------

Event: time 1420063746.165669, -------------- SYN_REPORT ------------
Event: time 1420063747.373693, type 4 (EV_MSC), code 4 (MSC_SCAN), value 90004
Event: time 1420063747.373693, type 1 (EV_KEY), code 275 (BTN_SIDE), value 1
Event: time 1420063747.373693, -------------- SYN_REPORT ------------
Event: time 1420063747.533689, type 4 (EV_MSC), code 4 (MSC_SCAN), value 90004
Event: time 1420063747.533689, type 1 (EV_KEY), code 275 (BTN_SIDE), value 0

How would one approach mapping these buttons to some virtual keys? For example, how would one map these buttons to act as back and forward ?