Update commands and README to fit `brew tap`

The changes in `brew tap` had a few extra side-effects that I hadn't
thought of. These updates account for those, hopefully without breaking
backwards compatibility of `brew any-tap` itself.
Merge pull request #5 from telemachus/compatibility-fix

Update commands and README to fit `brew tap`

Other `brew` tools assume a `homebrew-` prefix
as part of the naming-structure of taps. This fix
will keep `brew any-tap` in sync with the
expectations of the rest of the `brew` world.