Prepare the 0.0.24 release.

Bump version & complete the 0.0.24 CHANGELOG section.

Testing Done:
```console
$ ./build-support/bin/release.sh -n
[== Performing a dry run release - no artifacts will be uploaded. ==]
...
[== Building package pantsbuild.pants-0.0.24 with target '//src/python/pants:pants-packaged' ... ==]
...
[== Building package pantsbuild.pants.testinfra-0.0.24 with target '//src/python/pants:test_infra' ... ==]
...
[== Installing and testing package pantsbuild.pants-0.0.24 ... ==]
...
[== Installing and testing package pantsbuild.pants.testinfra-0.0.24 ... ==]
...
[== Dry run release succeeded. ==]
```

Bugs closed: 597, 606

Reviewed at https://rbcommons.com/s/twitter/r/1069/
Fixup backticks embedded in a here document.

These were intended to be in the context of a comment
but really were being evaluated.

Testing Done:
Saw the error before this fix here: https://travis-ci.org/pantsbuild/pants/jobs/36065429 - note `./build-support/bin/ci-sync.sh: line 51: .travis.yml: command not found`
```console
$ ./build-support/bin/ci-sync.sh
Syncing OSX CI to 0e3db04c45888b68689fb2fcbd1a4c3c658ef384 for CI build 2055.
Requirement already satisfied (use --upgrade to upgrade): PyYAML in /home/travis/virtualenv/python2.6.9/lib/python2.6/site-packages
Cleaning up...
./build-support/bin/ci-sync.sh: line 51: .travis.yml: command not found
[detached HEAD 9f425f7] Sync of https://github.com/pantsbuild/pants/commit/0e3db04c45888b68689fb2fcbd1a4c3c658ef384
 1 file changed, 13 insertions(+), 41 deletions(-)
 rewrite .travis.yml (92%)
Counting objects: 1900, done.
Delta compression using up to 32 threads.
Compressing objects: 100% (1062/1062), done.
Writing objects: 100% (1900/1900), 2.27 MiB | 0 bytes/s, done.
Total 1900 (delta 608), reused 1896 (delta 607)
To https://github.com/pantsbuild/pants-for-travis-osx-ci.git
 + 2b75951...9f425f7 HEAD -> master (forced update)
```

Bugs closed: 605

Reviewed at https://rbcommons.com/s/twitter/r/1070/
Payload refactor
Make internal_backend plugins 1st class local pants plugins.

This moves wiring-up of the pants_internal backends to pants.ini
and make the (future) role of src/python/pants/bin:pants clear
and distinct from src/python/pants/bin:pants_local_binary.

It also fixes some broken docsite.json paths and adds some
TODOs to nuke 3rdparty/python/dev_only_requirements.txt.
Reduce change scope - leave repos & indices in-place.
Fixup missed src/python/pants/bin:pants ref.
Fix sloppy parital move to pants_local_binary.
Clarify the role of src/python/pants/bin:pants
Fix typos
Fixup pants self-publish dogfood to use a controlled plugin env.
Merge branch 'jsirois/internal_backend/cleanup' of github.com:jsirois/pants into pl.payload_refactor_rebase

Conflicts:
	pants.ini
Fix utf8 reflection test