Switch from to branches for configurations

[fowles]

The existing setup with subdirectories lacks the simplicity users want from a fire and forget start project.

https://github.com/fowles/abseil-hello

Switches to a structure where each branch contains only the information needed for that build system. Major changes:

• cmake now includes googletest
• cmake and bazel build the same code and tests
• neither includes extra subdirectories
• all the README's have been reworked

Because this requires adding several branches, the ordinary "pull request" workflow doesn't work so I am just pointing to a finished repo for you to pull from.