Rename pip.vendor.* to pip._vendor.* to ensure it's obviously private
Merge pull request #1246 from dstufft/make-explicit-vendor-private

Rename pip.vendor.* to pip._vendor.* to ensure it's obviously private
improved cookbook entry for "Requirements Files"
changed the `pip bundle` deprecation notice so that:
- it says when the feature became deprecated
- for now, it doesn't specify when it will be removed until we're sure
- logs as a warning (with yellow color)
details on logging for users, but also for maintainers
Merge pull request #1248 from qwcode/logging_docs

details on logging for users, but also for maintainers
Merge pull request #1247 from qwcode/req_docs

improved cookbook entry for "Requirements Files"