Rename function, replace magic constant, remove atoi

rename queryParametersFromUrl to extractQueryParameters
'?' in extractQueryParameters function replaced by ParameterList::QUERY_SEPARATOR
use stringstream instead of atoi
Get rid of ParameterList getParametersAsMap method

in ParameterList: remove getParametersAsMap method, add getFirst, contain methods
parseSingleLevelJSON now returns OAuth::ParameterList
make corresponding changes to OAuth 2.0 Service class
Modify Service makeRedirectUrlParameters

use ParameterList
make redirectUri optional
Replace magic strings in OAuth 2.0 Service with constants

add HEADER_AUTHORIZATION constant to HttpRequest
replace raw strings in Service class with constants