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.
user site fix for windows tests; was broke recently when the 2 TestPipEnvironment classes were consolidated into one
merge from release-1.4 branch
--pre flag should affect the finder and not the requirements

Storing the --pre flag on the finder enables easily being able
to have it affect all package discoveries made with that finder.
The previous method of passing it into the InstallRequirement
meant that only top level dependencies were controlled by
--pre
Merge pull request #1040 from pypa/use-pre-for-dependencies

Fixes #1038 --pre flag should affect the finder and not the requirements
fix regexp for windows paths
merge from release-1.4 branch
patch for our tests cache for windows/py2
merge from release-1.4 branch
deprecate `pip bundle` and support for installing pybundle files
pip-1.4rc4
have pip release branch test using virtualenv release 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.
pip.index: fix crash getting Content-Type header

Returned headers may not always have the "Content-Type"
field available.  Avoid KeyError exceptions by using
dict.get() instead of blindly grabbing the header value.

Closes #32