Add HMAC-SHA1 and base64 algorithms
Add Signature class with tests

Implemented methods: PLAINTEXT, HMAC-SHA1
Add ParameterList::asMap method

It's needed to extract parameters from ParameterList and add them to
Authorization header
Minor changes and fixes in ServiceConfiguration, Token, HttpRequest

ServiceConfiguration: added callbackUrl parameter
Token: added getSecret() method
HttpRequest: getBaseStringUri() now returns encoded url
Add signRequest() method to Service class

Also added several helper methods and OAuth constants
Change Signature methods to take parameters
Fix Service class issues

Replace hardcoded signature method, add OAUTH_DEFAULT_VERSION constant,
remove all code related to header generation.
Replace ParameterList's asMap() method with asAuthorizationHeader()
Change Signature class to allow the user implement his own signing
method
Rename utility sources and move them into separate folder
Move OAuth-related functions from ParameterList to Utility

Methods ParameterList::asBaseString() and
ParameterList::asAuthorizationHeader() are free functions in Utility
namespace now.
Use new for syntax in iteration loops
Merge pull request #15 from Melzas/add-signature

Add Signature class and related stuff