Add rtc crystal

Add crystal for rtc. Cylindrical can is chosen. Not soldered to ground, as some manufacturers (e.g. http://www.abracon.com/Resonators/AB26TRB.pdf ) recommend against it. 
Capacitors were calculated for 50 k ESR, 12.5 pF Cl, 1.35 pF Cs crystal, assuming 5 pf of stray capacitance.