Merge branch 'lazy-reproducible-builds'

We have "lazy reproducible builds" now. What I mean by that is, that
the resulting "apk" archive is not fully reproducible, but all binaries
inside it are. This is necessary to kick-off the binary repo, which is
in turn required to get the testsuite going on Travis. Read #64 for more
information.

Usage:
```
pmbootstrap build hello-world --buildinfo
pmbootstrap challenge /tmp/path/to/hello-world-1-r2.apk
```

The "--buildinfo" parameter generates a "buildinfo.json", which contains
the versions of all dependencies. It is not very optimizied, so this
is a performance bottleneck and takes 10 seconds (which is quite much
considering that the hello-world package builds in less than a second).
This can be improved in the future, and then the buildinfo parameter
may become the default.

pmbootstrap.py

@@ -56,9 +56,12 @@ def main():
         if args.action == "aportgen":
             pmb.aportgen.generate(args, args.package)
         elif args.action == "build":
-            pmb.build.package(args, args.package, args.arch, args.force, False)
+            pmb.build.package(args, args.package, args.arch, args.force, False,
+                              args.buildinfo)
         elif args.action == "build_init":
             pmb.build.init(args, args.suffix)
+        elif args.action == "challenge":
+            pmb.build.challenge(args, args.apk)
         elif args.action == "checksum":
             pmb.build.checksum(args, args.package)
         elif args.action == "chroot":