Add const modifiers to some methods of Service class

getCallbackUrl, generateNonce, generateOAuthParameters
Split Service::signRequest into internal end external versions

external version signs provided HttpRequest
internal version adds specified additionalOAuthParameters and signs request after it
generateOAuthParameters now adds only parameters that are common for all requests
Implement credentials exchange

implement exchangeToken method of Service class
closes issue #21
Add tests for credentials exchange and request signing
Merge pull request #24 from swat32exe/implement-credentials-exchange

Implement credentials exchange and request signing
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
Add linking with OpenSSL to build files

Flag --with-rsa allows to build the project with RSA-SHA1 signature
method. OpenSSL must be installed. Build script tries to find OpenSSL
library file and include directory. If it fails, these paths must be explicitly
specified with --openssl-libpath and --openssl-includepath build variables.
Add sha1 algorithm for use with RSA-SHA1
Add implementation of RSA-SHA1 signature
Add tests for RSA-SHA1 signature
Make buildable under windows

paths changed to windows defaults
added necessary library
fixed openssl library name suffix
LIBS in test configuration reordered
Fix rsa under windows. Remove openssl linking to OAuth library.

applink.c included, it fixes OPENSSL_Uplink(10100000,08): no OPENSSL_Applink error
openssl linking to OAuth library isn't necessary, and sometimes will not work as might be expected
Merge pull request #3 from swat32exe/add-rsa-sha1-signature-melzas

Make buildable under windows
Use conditional compilation for openssl/applink.c and add libcrypto

openssl/applink.c is needed only on Windows
Fix possible memory leaks and change exception messages
Add Service tests for RSA-SHA1 signature