datadog: initial plog4 support work

- The plugin is now easy to test with:

        $ PYTHONPATH=~/.datadog-agent/agent ./plog.py

- The logic is now in the order keys are returned by plog.
- Our helper names follow the datadog naming convention.
- The Kafka logic is gone as Kafka is merely a handler now.
- Handlers are fully supported, with the following design:
  - If the same handler appears multiple times in the pipeline,
    we sum-merge stats from all instances.
    Note that handler instances could return different names to
    avoid this, if it ever really makes sense, as naming is dynamic.
  - Nested dictionaries are supported by compiling paths in the form
    'key1.key2.key3.'[...]
  - As we walk the tree, as soon as we see a non-dictionary, we switch
    to "nested summing", so [1,[2,{a:3,b:'c'}] = 1+2+3 = 6.

Closes #58.