changed 'PI' to 'Pi'; that's the official sp.
install instr. updated w/ prebuilt node, mongo

Added instructions to install Node.js and MongoDB from prebuilt
packages; updated regular instructions. See ni-c/heimcontrol.js#92
[GHPages] install instruction for python3 users

Additional instruction for python3 users (i.e., pretty much everyone), because `gyp` hates python3 and makes no attempt to find python2.