Move all scripts into bin

I've added ./bin to PATH, so this simplifies setup of multiple machines.
New script for installing standard Ruby gems

I'm sick of finding a bunch of important gems aren't available every
time I setup Ruby. It's affecting Vim (ctags and navigation into bundled
gems, in particular) and tmux.

Well, no more. When a new gem becomes part of my default environment,
this script will quickly take care of installing it the current version
of Ruby (as managed by rbenv), on any machine.