Wharfie

[#16] TOOLCHAIN commands are executed in target instead of host sysroot instead
- introduced post commands for host and target
- added tests for this bug

[#1] Build as non-root user
- use qemu to make non-root build
- needed host tools during build of qemu: all local wharfie tools and qemu-img
- needed host tools when building w/ qemu: tar, python
- biggest limitations:
--- The size of the possible input and output is hardly limitted.
--- The maximum output size is always fully allocated on disk

[#10]
- Support building of toolchains in one run.
- fixed build abortion
- fixed double creation of output file for temporary targets

- check for the tool kpartx
- made build more verbose, and included the hash in its output

--info didn't work anymore. fixed that

[#8] experimental implementation of incremental/multi-level build.
Actually it seems as tar has still problems with it. I can't avoid that tar thinks that s.th. is renamed, even if it is not.
In my tests I found no problems, except the warning from tar until now. But I added it as a command line option anyway, to be safe.

By-the-way I fixed the handling of files in ADD a bit.
- URLs are supported now
- Implicite makerules don't interfer anymore with ADD

[#6] Support downloading files

[#4] abort on error

restructured the main python script into submodules (curently only makefile generation and wharfile actions)