Pin installed formulae.
See mxcl/homebrew#18386.
Added `pin` et. al. to manpage.
* Added `brew pin` to `brew.1`
* Added `brew unpin` to `brew.1`
* Added `brew list --pinned` to `brew.1`
* Added information about frozen formulae to `brew upgrade` in `brew.1`
Added `pin` et.al. to completion scripts.