Egginst: Overwrite symlink on egg installs.

Prior to symlink changes, you could egginst a new/old egg version
over an existing egg install, which was broken in eggs having
symlinks. This commits makes them work again by overwriting
existing symlink instead of failing on them.
Merge pull request #82 from pankajp/egginst-overwrite-symlink

Egginst: Overwrite symlink on egg installs.