Create Extension XML filles distribution and publication system

Extensions have currently simple system of storing information - all the necessary info is in there. The goal is to provide centralised place for publishing and retrieving those files (but not actual extensions). Extension maintainer will be able to easily publish his/her extension's XML files on extensions server and by default, each CiviCRM installation will be reaching to this server for information about what extensions are available, what are the latest versions, etc. In next version, there should be an ability to build an extensions directory (website) based on that.