Rearrange elixir code, update README

    mix escriptize

executed in the elixir/mandelbrot directory will generate a script which
can be run as

    ./mandelbrot

This means that Mandelbrot.main/1 should be the main routine, so I moved the
pixel coordinate stream generator into a more appropriately named module.

Minro change, to be compatible with the Ruby version the pixel coordinates
run from 1 .. limit in each dimension.