Enable HttpRequesTask to make POST requests

This PR allows the HTTP method to be set as POST (rather than the default GET) and for post parameters to be added as per the documentation of HTTP_Request2 at http://pear.php.net/manual/en/package.http.http-request2.request.php

An example of how an http-request task could now look: 

<http-request
      url=""
      authUser=""
      authPassword=""
      verbose="true"
      method="POST"
      >
      <config name="adapter" value="HTTP_Request2_Adapter_Curl"/>
      <header name="user-agent" value="Phing HttpRequestTask"/>
      <header name="content-type" value="application/json" />
      <postparameter name="param1" value="test1" />
      <postparameter name="param2" value="test2" />
    </http-request>

Future patches could add support for other HTTP methods.