Download CEF builds from S3

Story https://trello.com/c/1JK3Ordd

Changed the download URL to get the CEF builds from S3 instead from dev.brackets.io.
Added task to download the symbol files for the CEF version that we use to build Brackets.
I wanted to write the symbol files in it's own subdirectory, so that it doesn't mess up the `brackets-shell`. I can put them wherever it makes the most sense. Perhaps I put them in `cwd()/deps/cef/release/` for both platforms.
I wanted to change the script to download the release and debug version at the same time for windows. Or do you want different grunt tasks for that?