Incorporate feedback from core team.

[dhermes]

After porting the Mercurial history to git, I was able to convert the old project to a modern Python / GitHub / Travis open source project in six commits:

• 7b6c5c9: Remove old cruft, document the Mercurial history migration. Make README more GitHub friendly.
• 01da236: Actually make the test runner work and turning on Travis. Also documenting new setup for tests.
• 35b6841: Re-organizing files and folders of project by logical purpose. Also updating Makefile to actually work for most targets (was mostly broken). Files here were only moved / renamed to make git history easier to grok.
• 3e9c3aa: Fixing test modules to make Makefile test targets work.
• bdc9849: Switching from ad-hoc test runner module to using tox and nosetests for unittests and for test coverage. Also turning on Coveralls and adding badge to this project.
• 760cf89: Removing "code quality" type targets from Makefile and implementing lint rule for tox. Also cleaning up code that violated the pep8 CLI tool.

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I didn't add this to the documentation (but should have). Getting set up for local dev is as easy as running

./get_app_engine_sdk.sh
export GAE="./cache/google_appengine"

(To support Windows users I should convert the bash script to pure Python.)

[dhermes]

Should also probably rename the project ndb-git -> appengine-ndb. This will require updating a few docs:

$ git log -1 --pretty=%H
760cf89c9245cbfca1c119ccd0446f4f9c0e63f0
$ git grep ndb-git
CONTRIBUTING.md:    https://github.com/dhermes/ndb-git/
CONTRIBUTING.md:    $ git clone git@github.com:USERNAME/ndb-git.git hack-on-ndb
CONTRIBUTING.md:    # Configure remotes such that you can pull changes from the ndb-git
CONTRIBUTING.md:    $ git remote add upstream https://github.com:dhermes/ndb-git
CONTRIBUTING.md:    the current working directory is the `ndb-git` checkout directory:
README.md:[![Travis Build Status](https://travis-ci.org/dhermes/ndb-git.svg?branch=master)](https://travis-ci.org/dhermes/ndb-git)
README.md:[![Coverage Status](https://img.shields.io/coveralls/dhermes/ndb-git.svg)](https://coveralls.io/r/dhermes/ndb-git?branch=master)
README.md:mkdir ndb-git && cd ndb-git
setup.py:    url='https://github.com/dhermes/ndb-git',