fixed bug where --always-copy cannot copy executable when --python is us...

When --always-copy is used in conjunction with --python=python2.6 (or --python=/usr/bin/python2.6), virtualenv was unable to copy the python binary (rather than symlinking it).

This occurred on osx 10.7.5, although it didn't look like a platform-specific issue.

This change fixes this as well as copying the file permissions.