Freeze installed formulae.

See mxcl/homebrew#18386.
- [x] Pinning/unpinning formulae
- [x] Pinned formulae don't upgrade
- [x] Forcing upgrade of pinned formulae by named argument to `upgrade`
- [x] Documentation of the pinning feature
- [x] Unpin formulae on uninstall
- [x] Move pin symlink to new version on upgrade (or: symlink to `opt/<formula>´)