On OS X, use the homebrew install of GNU readline instead of editline


There are a bunch of errors when trying to get readline on Mac OS X:

``` console
$ go get -u github.com/bobappleyard/readline

error: 'rl_catch_sigwinch' undeclared (first use in this function)
error: (Each undeclared identifier is reported only once
error: 'rl_catch_signals' undeclared (first use in this function)
error: 'rl_resize_terminal' undeclared (first use in this function)
```

This is the same editline nonsense as #1.

To resolve this issue, this pull request utilizes the [Homebrew](http://mxcl.github.com/homebrew/) install of GNU readline.

>OS X provides the BSD libedit library, which shadows libreadline.
In order to prevent conflicts when programs look for libreadline we are
defaulting this GNU Readline installation to keg-only.

>Generally there are no consequences of this for you. If you build your
own software and it requires this formula, you'll need to add to your
build variables:

>    `LDFLAGS:  -L/usr/local/opt/readline/lib`
    `CPPFLAGS: -I/usr/local/opt/readline/include`