Add section to download cURL in Windows installer

[brad-anderson]

@WalterBright This pull request relies on curl-7.24.0-dmd-win32.zip being uploaded to ftp://ftp.digitalmars.com/ before it should be merged.

[andralex]

@eco, @WalterBright: that won't fly as Walter's build and upload procedures are manual and extremely error-prone as they are. Walter did introduce some automation but it's his own scripts that are not in github. We need to have Walter commit to github his scripts, and then update the scripts to upload the curl lib.

[brad-anderson]

@andralex It's just a one time manual upload he'd need to do. I'd like to switch all the download links over to use the Github URL since it's several times faster but figured that was a different issue that could be addressed separately.