As opposed to a flat directory, we now have the following structure:

- security
  -context
  - credentials
    - composite
    - fake
    - google_default
    - iam
    - jwt
    - oauth2
    - plugin
    - ssl
  - transport
  - util

We have not refactored the test code yet but this PR is already large
enough...

-) Introducing gpr_vlog so to spare a few vsprintf later (at least one for now)
-) Renaming statistics/log.* to statistics/census_log.* to avoid collisions.
	Change on 2014/12/12 by nnoble <nnoble@google.com>
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Created by MOE: http://code.google.com/p/moe-java
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