CI + bunch of fixes

[AkihiroSuda]

.travis.yml:

• Newly added (Fix #40, let me know if you prefer other CI :smile:)

.mailmap, AUTHORS, Makefile:

• Fix make AUTHORS, which was broken because of lack of .mailmap (I can remove this target if unneeded)
• Skip running lint by default because it is too noisy about "comments are missing"
• Remove wrong ${DOCKER_BUILDTAGS}
• Fix make binaries, which was just an empty target

README.md:

• Add godoc badge and travis badge

digests_test.go, resource.go:

• Fix for go vet

manifest_test.go, resource_test.go, testutil_test.go:

• Add missing diffResourceList() from #39 (@dmcgowan)
• Remove dependency for tree(1)
• Fix for go vet
• Fix the following test failure;

  
  manifest_test.go:163: Resource list difference
  manifest_test.go:171: Changed resource:
                  Expected: &continuity.symLink{resource:continuity.resource{paths:[]string{"/c/a-abssymlink"}, mode:0x80001ff, uid:"1001", gid:"100
  1", xattrs:map[string][]uint8(nil)}, target:"/a"}
                  Actual:   &continuity.symLink{resource:continuity.resource{paths:[]string{"/c/a-abssymlink"}, mode:0x80001ff, uid:"1001", gid:"100
  1", xattrs:map[string][]uint8{}}, target:"/tmp/continuity-test-356664233/a"}

Now CI passing: https://travis-ci.org/AkihiroSuda/continuity/builds/208498783

I'm also planning to do vendor using vndr in another PR.

[dmcgowan]

Thanks for getting all this in shape! Just a couple small changes on the build process and we can get this merged.

[AkihiroSuda]

done @dmcgowan

[dmcgowan]

LGTM

I think we may want to move some of the test functions around in the future. Maybe to a subpackage so they can be imported by other packages.

[dmcgowan]

@stevvooe going to merge this, if you want further changes we can do in follow up