Revert "Merge pull request #1519 from dstufft/remove-dependency-links"

This reverts commit da02f073dabdc9e72733847aa1727ec19869109d, reversing
changes made to 2ad8888901c041b8f9aacebb3a16d55bf631e867.

Conflicts:
	CHANGES.txt
	pip/commands/freeze.py
	pip/commands/list.py
	pip/commands/wheel.py
	pip/index.py
	pip/req/req_set.py
	pip/vcs/subversion.py
Merge pull request #1955 from dstufft/revert-dep-links-removal

Revert "Merge pull request #1519 from dstufft/remove-dependency-links"