Add 'show' as alias for 'info' in brew.rb

Every day I see this at least once:

```
$ brew show something
Error: Unknown command: show
```

ARRGH! Please ... for those of us from a Debian / Ubuntu (i.e. apt-get) background who are therefore unavoidably synaptically hard-wired for eternity to type "brew *show* ..." rather than "brew *info* ...", can we please add an alias for this to the brew.rb file?

I beg you kind sirs, accept this change (or suggest an equivalent)!

Signed, your most humble and long-suffering brew mendicant ;)