Updated error message reported when `--allow-hosts` blocks a link to provide a less startling user experience. Fixes #71.

Powershell isn't installed by default on Windows Server 2008. Also it's possible that Powershell is present on older systems. Now use direct detection of a Powershell executable to determine viability of that technique for a downloader. Fixes #67.

Use jaraco.packaging 2.0 to invoke pushing the bookmark as a separate operation from pushing the other changesets.

Removed the --delete-conflicting and --ignore-conflicts-at-my-risk options, which have long been deprecated and indicated that they're not used.

Changed the launcher relevant environment variable to SETUPTOOLS_LAUNCHER and it now accepts different strings 'natural' and 'executable', instead of only reflecting a boolean value.

Updated changelog once more to provide a re-assuring narrative about the backward-incompatible changes.

--HG--
extra : histedit_source : a1c329f92ee71a707955e352b63987af9de6e120

--HG--
extra : histedit_source : ef811b339d08652c845c5556fd7e2d0f38ee12df

Allow the test to fail on its own merits rather than failing with a not-so-useful message; removed Python 2.3 support.