Implement pluggable discovery adapters and standalone tests

This pull request adds a `Locator` interface for server discovery, extracts the etcd discovery implementation into an `EtcdLocator`, and adds a `StaticLocator` adapter. The config loading mechanism has also been refactored, making it possible to run the tests without etcd, memcached, or a standalone Simple Push server.

* The router now uses a buffered channel (`bucket_size`) instead of partitioning contacts into buckets.
* The etcd key prefix and minimum refresh interval are exposed as `EtcdLocatorConf.Dir` and `.RefreshInterval`, respectively.
* `EtcdLocator` now uses the normalized key (prefix + hostname) consistently, and accounts for non-default HTTPS ports when constructing the key.
* The `PluginLoaders` map associates plugin types (i.e., logger, proprietary pinger, metrics, storage adapter, router, locator, server, and handlers) with initialization functions. This is used by `TestServer` to specify configuration options programmatically. The `*Conf` structures have also been flattened where appropriate.
* The `Host` and `Port` options have been consolidated into `Addr`. If `Addr` is omitted, the server and router will listen on random ports.
* `Server` and `Router` now initialize their own `net.Listener`s, avoiding a race condition where the tests would begin running before the server was ready.

Closes #30. @bbangert, @jrconlin, @rafrombrc r?
I'm also thinking that if we're going to treat these like plugins, we should build all loadable items as plugins. (full interfaces rather than known pointers like this). That can be a follow-up bug.