Proposed fix to #1180 - Make 'pip search' support http proxy

This patch is my two cents trying to fix the unsupported --proxy option for the search command. Following changes have been made:

  * Make use of `--proxy` option in `search` command.
  * Add a new `UrllibTransport` class for `xmlrpclib.ServerProxy` to support  http proxy based upon `urllib` (Py3) / `urllib2` (Py2).