Modify HttpRequest to add capability to store arbitrary body

body is now represented by <code>string</code>, not by <code>ParameterList</code>
<code>Content-Type</code> header is added automatically when <code>HttpRequest</code> is created and is set to <code>application/x-www-form-urlencoded</code>
add <code>extractBodyParameters</code> function to <code>utility/Extractor.h</code>, that extracts body parameters, make corresponding changes to <code>Service.cpp</code>
make corresponding changes to tests, add test, where request with arbitrary body is signed