appc/spec: make version a first-class required field
stage1: lessen systemd-nspawn dependency on host runtime

 * Introduces "/fakesdboot.so" shim for intercepting lstat of
   "/run/systemd/system" in stage1 rootfs
 * LD_PRELOAD=fakesdboot.so in stage1 systemd-nspawn execution
 * LD_LIBRARY_PATH=usr/lib of stage1 so we use stage1 libs
 * explicitly use stage1's ld-linux interpreter in executing
   systemd-nspawn

 It would be tidier to just chroot into stage1's rootfs before
 executing /init, but then we'd need to wrangle the bind mount
 source paths into the chroot beforehand.

 I'm leaning towards just rolling the systemd-nspawn
 functionality we actually desire into the statically-linked
 /init and remove systemd-nspawn from the bootstrap altogether as
 the next step.
Merge pull request #149 from vcaputo/fake_sdboot-use_our_libs

stage1: lessen systemd-nspawn dependency on host runtime
appc/spec: address comments
examples: remove old downloadstore example
Merge pull request #180 from jonboulle/downloadstore

examples: remove old downloadstore example
Merge pull request #177 from jonboulle/spec2

appc/spec: make version a first-class required field
appc/discovery: update test to new discovery
cas: update tests
appc: add version to ace manifests
Merge pull request #184 from jonboulle/fix_aci

appc: add version to ace manifests
mkrootfs.sh: add flags for dlsym

-Wl,--no-as-needed flags for gcc to avoid issue with dlsym
Merge pull request #191 from ghigt/fix-flags

mkrootfs.sh: add --no-as-needed linker flag to fakesdboot.so build
Merge pull request #181 from jonboulle/tests

*: fix up broken tests
stage0: clear up rootfs unpacking error propagation
stage0: support uncompressed (tar) rootfs images
Merge pull request #197 from jonboulle/err

stage0: clear up rootfs unpacking error propagation
tests: add travis-ci config and test script