Fix build script to use default web proxy to download remote files.
Update formatting of PATENTS.TXT
Merge branch 'build-script-web-proxy' of https://github.com/PatrickMcDonald/corefx into build-script-web-proxy
[Closes #44] Fix spelling error in ImmutableStack

- Fix spelling error in `System.Collections.Immutable.ImmutableStack`1` on Line 40