Move webmachine from riak_core.

Rebased #29 onto develop.

Generally,
    riak_core_web -> riak_api_web
    riak_core_wm_urlmap -> riak_api_wm_urlmap

... but with modifications:

* Listeners from riak_core_sup are now added in riak_api_sup.
* We are ignoring any `disable_http_nagle` setting. Nagle is off.
* Default SSL cert/key are scoped to the platform_etc_dir instead of
  the static "etc/".
* Formatted the listener names according to "SCHEME://IP:PORT". IPv6
  interfaces are surrounded with square brackets to conform with
  convention.