Modify HttpRequest to add capability to store arbitrary body

body is now represented by string, not by ParameterList
Content-Type header is added automatically when HttpRequest is created and is set to application/x-www-form-urlencoded
add extractBodyParameters function to utility/Extractor.h, that extracts body parameters, make corresponding changes to Service.cpp
make corresponding changes to tests
closes #25
Add test, where request with arbitrary body is signed
FORM_URLENCODED and HEADER_CONTENT_TYPE constants moved outside HttpRequest
Content-Type header initialization removed from HttpRequest constructor
Merge pull request #26 from swat32exe/modify-httprequest

Modify HttpRequest to add capability to store arbitrary body