Encode formula revision in installation prefix

In order to allow kegs built with the same version but differing
formula revisions, we must encode the revision as part of the keg's
name. This is necessary to actually *do* the upgrade, as we cannot
upgrade a keg in-place, and temporarily moving it pending the result of
the upgrade is error-prone and potentially slow.

To accomplish this, we tack the revision onto the end of the standard
keg name, preceded by an underscore (e.g. "1.0_1"). Formula#version
learns to append this revision when appropriate. An exception is made
for a formula that has no revision: the tag is omitted. This preserves
compatibility with existing kegs.