Fix build script to use default web proxy to download remote files.
Bring immutable collections library's block code coverage up to ~97.5%
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
Fixed inconsistent casing in filename references to match with what's on disk

This makes the solutions build with xbuild on Mono/Linux on a case-sensitive filesystem