Libxml2 XML wrappers now working.
Seek functionality for libzip in place at lowest level only, needs to be added to ByteStream classes.
Merge branch 'feature/WinRT' of github.com:readium/readium-sdk into feature/WinRT

* 'feature/WinRT' of github.com:readium/readium-sdk:
  Fix Stream::ReadAsync to set buffer lenght