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