Merge branch '1.3.X'
Merge branch '1.3.X'
Add a User Agent so that pip is identifiable in logs
Merge pull request #901 from pypa/dstufft/user-agent

Add a User Agent so that pip is identifiable in logs
improve method for getting home scheme purelib
Remove commented out code that was mistakenly left in
'irregardless', if it's a word at all, means the opposite of how it's meant here.
Merge pull request #905 from iguananaut/irregardless

'irregardless', if it's a word at all, means the opposite of how it's meant here
Merge pull request #903 from qwcode/home_lib

improve method for getting home scheme purelib
deal with pre-existing build dirs
pep8 comment fix
Merge pull request #865 from qwcode/no_reuse_build

deal with pre-existing build dirs
update changelog
Assume versions are a pre-release if we cannot parse them

* The most likely unparseable version is something like "dev" or
  "py6-dev" etc. The number of sane version numbers that are
  not normalizable and should be conisdered stable are fairly low.
Merge pull request #908 from pypa/dstufft/default-prerelease-true

Assume versions are a pre-release if we cannot parse them
update the release process
add link to pypa-dev list
Display hashdigest instead of hash object upon failure.
PreviousBuildDirError logging and test fix
PreviousBuildDirError test fix