Revamp Windows installer

**Don't download D2 files, instead embed them in the installer.** This means no internet access is required for the installer and the file size estimations should be correct now. Embedded folder is specified by the `EmbedD2Dir` define which can be specified by using `/DEmbedD2Dir=<dir>` to `makensis.exe`. I've changed `build_all.d` to do this automatically.

**Remove D1 and DMC logic from D2 installer.** They can still be downloaded and installed by the installer but they now use their own installers like Visual D does.
  - New installer scripts for D1 and DMC are included.
  - Installers for D1 and DMC should be generated and uploaded to one of the download locations (preferably downloads.dlang.org). They are made the same way D2's installer is made; by specifying version and a directory define to makensis. I could generate these and offer a download but it'd probably be more appropriate if someone more official like the release manager rolls them.
  - The URLs in the D2 installer script for these two new installers should match wherever they end up.

Also improves usage instructions and did an overall clean up.

I'd like to see libcurl included. It seems like the easiest way is to include it in the `localextras` zip @Abscissa offers but I'm not entirely sure if that works (it seems some files are pulled from the existing dmd installation). What's the best way to do this?

I'm also not entirely sure if the changes to `build_all.d` were done correctly. I'm not in the position to actually test `build_all` at the moment. `create_dmd_release` can create a 64-bit Windows release (neat, didn't know it did that) but I only accounted for the 32-bit release directory. @MartinNowak or @AndrewEdwards, could you check my work here?

Precise uninstallation will have to come later. It turns out it doesn't easily work when you use `File /r` but there are workarounds I need to look into.
So just `immutable build_dir = "dmd."~ver~".windows/dmd2"` then?