keyczar.js: Default to 2048-bit RSA keys instead of 4096-bits.

Matches C++ and Python Keyczar. Shawn Wilden (Google) stated that Java should
also be using a default of 2048-bits:
https://groups.google.com/d/msg/keyczar-discuss/BLEY_Dp4_7U/GCViLsJPj2kJ