Bazel Maven Repository Rules v2.0.0 Alpha 4

All existing changes in prior alphas, plus:

• Unpacked .aars now don't just "deps" their unpacked classes.jar, they export them.
• Add a config option to set the default threadcount for metadata fetching.
• Add an env var mechanism to tune fetch threads. Set BAZEL_MAVEN_FETCH_THREADS to a positive integer.
• More careful filtering out of non-build-related dep scopes (i.e. system, which behaves weirdly with maven properties)
• Improved packaging handling in Kramer to recognize unhandled packaging types.
• Support include and deps configurations, to go beside exclude
  • include/exclude can work together, but not with deps which overrides the deps list, and build_snippet overrides everything.
• Handle aar's with /libs/*.jar
• Add a facility for overriding values via a settings.json file
  • presently only used to set mirrors to override repo urls, but may carry more per-environment configuration, such as auth credentials.
  • file location override-able with env var BAZEL_MAVEN_SETTINGS (for CI, or other cases of pointing at a different settings)

  • TODO: set a default (e.g. ~/.m2/settings.json)

• other minor fixes, cleanups, formatting.