Copy request parameters to Solr's response

This patch copies the request parameters (explicit ones only, not the defaults) to Solr's XML output.

It is not configurable yet, it is enabled by default and adds a "queryParameters" list to the responseHeader:

<responseHeader>
<status>0</status>
<QTime>1</QTime>
<lst name="queryParameters">
<arr name="multi">
<str>red</str>
<str>blue</str>
</arr>
<str name="rows">10</str>
<str name="start">0</str>
<str name="indent">on</str>
<str name="q">solr</str>
<str name="stylesheet"/>
<str name="version">2.1</str>
</lst>
</responseHeader>

The above example includes a multi-valued parameter, "multi".

This might still change a bit, but if someone wants to play with it or improve it, here you go.