updater can not be used without http libary

[bckmnn]

Hi there, it seems like updater can not be used without your http library.

downloader.cpp includes "http.h", which on my windows machine resolves to something from the windows sdk giving me a lot of compliation errors. Maybe you can add it to the readme.

[flaviotordini]

Thanks I'll fix it when I get back to a project using the lib. Or just submit a PR. Should be trivial. Search for ifdef HTTP in the code