Download, expand, write and verify the image file without caching it

enables installation on memory-constrainted machines and/or read-only boot
devices, and/or machines without any scratch space.

CoreOS installs now on VMs with 256M memory.

closes #124
Indeed, thanks. We have to take into account that failure can have this reasons:
• wrong input (we don't send an "Accept: …" header!)
• cannot process (bunzip2 fails, for example because it needs MOVBE instruction which the CPU doesn't have)
• cannot write

The error message will be displayed anyway. Therefore "expand" sounds diplomatic enough not to add confusion on one of the other cases.