Add HTTP module for downloading files

[vlukash]

No description provided.

[matthid]

Thanks a lot.
When we add a new Fake.<> namespace we need to add it here as well:

https://github.com/vlukash/FAKE/blob/1a8ba9ba009d94156a27982798ce9571ee113deb/build.fsx#L622-L631

This comment is more of a reminder for me before I merge this (but feel free to add it).

[vlukash]

@matthid got it, thanks

Updated build.fsx and added functionality for parallel downloading
Ready for review, please share your thoughts on API/improvements

Current API is :

Single file download

downloadFile (localFilePath: string) (uri: string) : Result<string, string list> 

Usage example

Http.downloadFile "5.zip" @"http://ipv4.download.thinkbroadband.com/5MB.zip" |> ignore

Download in parallel:

downloadFiles (input: DownloadParameters list) : Result<string list, string list>

Usage example

let files: Http.DownloadParameters list = [
    {Path = "5.zip"; Uri = @"http://ipv4.download.thinkbroadband.com/5MB.zip"};
    {Path = "10.zip"; Uri = @"http://ipv4.download.thinkbroadband.com/10MB.zip"}]
let result = Http.downloadFiles files
match result with
    | Ok files ->
        printfn "%A" files
    | Error err ->
        printfn "%A" err

[matthid]

My thoughts are:

• It's good to have this ;)
• I'd prefer throwing exceptions by default, because I think that is what other APIs are doing
• We can have this as is but I think I add a try prefix to the function names
• Namespace and module are fine (I like Fake.Net.Http)

The only other thing is if we want to have some docs for this, but we can improve this along the way.
Thanks a lot.