fix libCurl download

- Use same URL/zip as windows installer.
- get_dlibcurl32 fails, because the downloaded libcurl.dll
  depends on additional dlls.
- Use std.net.curl to download the zip, i.e. create_dmd_release
  depends on a working libcurl.dll, but uses the downloaded one
  to build a release.

This as well as #28 wait for someone to upload [libcurl-7.34.0-WinSSL-zlib-x86-x64.zip](http://d.darktech.org/libcurl-7.34.0-WinSSL-zlib-x86-x64.zip) to http://downloads.dlang.org/other/,