Add os.path.expanduser call to normalize_path.

realpath by itself won't expand tildes, which could lead to things like:

pip wheel --wheel-path=~/wheels -r requirements.txt

Producing:

Destination directory: /home/foo/src/dir/~/wheels

Which is pretty much never what you wanted.
merge from release-1.4 branch
merge from release-1.4 branch
merge from release-1.4 branch
merge from release-1.4 branch
Versioned commands consistent with python.

Fixes #1050
Fix functional test.
Default to disallowing external and insecurely hosted files
Fix tests with the new defaults
Merge pull request #1053 from pnasrat/1050-pep439-versioned-commands

Versioned commands consistent with python.
Merge pull request #1055 from pypa/default-externals-to-off

Default to disallowing external and insecurely hosted files
Merge pull request #1028 from emonty/develop

Add os.path.expanduser call to normalize_path.
merge from release-1.4 branch
Merge branch 'release-1.4' into develop
Merge branch 'release-1.4' into develop

Conflicts:
	pip/__init__.py
Don't ignore already installed pre-releases
Merge pull request #1075 from dstufft/always-consider-installed

Don't ignore already installed pre-releases
Don't ignore already installed pre-releases
Test the new branch name
Test the new branch name