Riak 2.0: Add Security

To set up the cluster you can:
- export RIAK_DIR=`<path to riak>`
- cd `buildbot`
- `make preconfigure_security`
- Start cluster
- `make configure_security`
- `make test_security`

Not sure if all of these have been sufficiently addressed, but please comment if not:
- [x] Investigate python stdlib ssl vs. python-gnutls, perhaps write a compatibility layer
- [x] Configure auth credentials, certificates, keys, cipher suites on client
- [x] PB connection negotiation (ref basho/riak_api#35)
- [x] Force HTTPS if auth provided
- [x] Handle errors from connecting to mismatched server (i.e. different expectations of security)

Failure on two tests because M/R and security is currently broken.  Issue tracked in https://github.com/basho/riak_kv/pull/972