This PR does the following:
* fix tests -- now, they're always built in the CMake scripts, and they work on VS2015
*add a new flag, BUILD_TESTING, which allows one to turn off testing builds
* Add documentation for running tests
* Add detection for VCPKG_DOWNLOADS environment variable in vcpkgpaths.cpp.
* Pass the downloads directory from VcpkgPaths to cmake.
* Also fixup bootstrap on *nix.
* Make error message a little prettier.
* Make that bash script actually work :)
* [vcpkg] Alter Optional<> usage style
* [vcpkg-docs] Add section on Environment Variables to the docs
The current names and titles of the examples put too much emphasis on the package which is being demonstrated rather than the concept. This can be jarring for newcomers. This patch renames them to put the emphasis on the concept instead.
