Add battery charger circuit

Thank you, Gordon!
Hah, silly me. I completely overlooked the case when uUSB is not plugged in... I shouldn't try to charge the battery with its own power...

OK here's updated list to do:

• Mount MAX1555 on SMD proto area
  
• Wire BAT (MAX1555) to BAT_IN (Espruino's JST PHR-2 2)
  
• Wire USB (MAX1555) to USB_VBUS (access at D1 1N5819)
  
• Connect GND (MAX1555) and GND (Espruino)