erlang: Client refactor, part 1

erlang: Client refactor, part 1

- Client is no longer a separate process.
- Simplified constructors moved into another module.
- All functions and exceptions return the new client,
  to allow for future statefulness in the client.

NOTE: With the new library and old gen-code, attempting to call a
nonexistent function will result in an exit.

TODO: fix docs and tests (tether test is not meaningful)

git-svn-id: https://svn.apache.org/repos/asf/incubator/thrift/trunk@990979 13f79535-47bb-0310-9956-ffa450edef68