The big rename!

Docker hub repo:
    stephenthirlwall/rpi-xc ->
    sdt4docker/raspberry-pi-cross-compiler

Github repo:
    sdt/docker-raspberrypi-cross-comper ->
    sdt/docker-raspberry-pi-cross-compiler

Script:
    rpi-xc -> rpxc

Binary prefixes:
    rpi- -> rpxc-

Image directory:
    rpi -> rpxc

Config file:
    ~/.rpi-xc ~/rpxc

Environment vars:
    RPI_XC_ -> RPXC

Closes #7