Move requests-logging setting to global scope

This should happen at import, since the __init__ may have been run in a different proc, and the cache pickled, since then.

Testing Done:
https://travis-ci.org/pantsbuild/pants/builds/38522554

Reviewed at https://rbcommons.com/s/twitter/r/1022/
The traversal order used is preorder, not inorder; support postorder traversal

1. The term "inorder traversal" does not make sense outside of the context of binary trees.  Instead, use the term "preorder", which is actually what is done.

2. Also allow for postorder traversal (and test that it works).

Postorder stuff is needed for https://rbcommons.com/s/twitter/r/1179/

Testing Done:
./pants goal test tests/python/pants_test/graph:graph

Reviewed at https://rbcommons.com/s/twitter/r/1177/
Handle list-typed options (action='append') correctly.

Ensure that we read them from config in the right way, and apply type
conversion to the list members.

Added test cases for this.

Testing Done:
Ran the options unittests before and after the fix, to verify
that the test discriminates this.

CI passed.

Reviewed at https://rbcommons.com/s/twitter/r/1183/
convert and move about 1/2 of our docs from .rst to .md

This change starts converting our docs from sphinx-powered-rst to Pants-enhanced-md, moving files to some different directories:

src/docs/*.md  <= language-agnostic Pants-user docs

examples/src/java/com/pants/examples/*.md  <= Pants-with-Java docs

examples/src/python/example/*.md  <= Pants-with-py docs

src/python/pants/docs  <= good place for pants-dev docs

Keeping foolang docs with examples/src/foolang feels right; if we change the examples, we probably want to change the docs, too. Less-certain about where language-agnostic docs go.

Testing Done:
eyeball-compare pages gen'd from .rst, .md

Reviewed at https://rbcommons.com/s/twitter/r/1176/
'goal sitegen' tweaks inspired by eyeballing more Pants doc pages

Don't insert pilcrow anchor in the heading; instead, lay it nearby with a table. Otherwise, it looks funny if the line breaks right there.

Add a "Generated by publish_docs dist/markdown/html/README.html 2014-10-17T11\:16:07.253750" so that folks who are looking at a doc from two years ago have an inkling of the danger. And/or if they want to fix a mistake, they have a clue where the content came from.

Widened margin around the page-level table of contents.

Testing Done:
https://travis-ci.org/pantsbuild/pants/builds/38290410

Reviewed at https://rbcommons.com/s/twitter/r/1178/
Make CacheKey hash and pickle friendly

Don't need payloads on cache_keys, so removing them makes them pickle-friendly.

Testing Done:
tests/python/pants_test/cache::

Passed: https://travis-ci.org/pantsbuild/pants/builds/38538962

Reviewed at https://rbcommons.com/s/twitter/r/986/