Tweak and Refactor Pry::Terminal

The biggest behavior change is to favor `io/console` (but still not when
on jruby). The previous favorite was `Readline.get_screen_size`, but
that thing is stale just like the env. The only reason it gets updated
is if you put `Pry.auto_resize!` in your `~/.pryrc`

The next change is more error handling in `Pry.auto_resize!`

Otherwise I renamed `TerminalInfo` to `Terminal`, which is something
andrewvos and I discussed way back when `TerminalInfo` first came about.

The rest was a little Extract Method stuff, no biggie.