unpack_url: session only needed for unpack_http_url
vcs: add documentation to VersionControl class
cleanup: use "with open()" to open files
freeze: simplify code and use with open()
Merge pull request #2034 from xavfernandez/xfernandez/small_improvments

Small improvements
remove tag_svn_revision to fix test, same as PR #2045
Merge pull request #2116 from bukzor/buck-remove-tag-svn-revision

remove tag_svn_revision to fix test, same as PR #2045
Use a secure randomized build directory when possible
Merge pull request #2122 from dstufft/randomized-build-directory

Use a secure randomized build directory when possible
Psycho-rebased branch feature/freeze-subset-recursively on top of develop.