renamed plugins to contrib (they are not really plugins)
rewrote README.md to reflect new usage
added new dir to setup.py so it gets copied with installation, in views
of making using inventory scripts easier in teh future

using newly added `EnvironmentConfig` class in `serfclient`.

See https://github.com/KushalP/serfclient-py/pull/17

This makes the Serf inventory plugin use the `SERF_RPC_ADDR` and
`SERF_RPC_AUTH` environment variables that the `serf` command-line tool
already uses.

These can be used to get Serf data from a remote node instead of
requiring the ansible control host to be running a serf agent and to be
a member of the serf cluster.

Add inventory plugin for [Serf](https://serfdom.io/).
Requires [`serfclient` Python module](https://pypi.python.org/pypi/serfclient).