Bash: change color of $PS1 + add hostname

The default text color of $PS1 is now white-on-light-blue. $PS1 also
now includes the hostname as part of it.
Bash: remove double-alias of `ls`

In the block of code from Ubuntu's default ~/.bashrc that we copied
into ~/.shell_aliases, there was an alias for ls that simply turned on
color. This was clobbering our own alias that turned on color and
enabled long listings and hidden files. The Ubuntu alias has been
deleted.
Bash: add (empty) ~/.bash_logout

This file is sourced by Bash before every logout action. Ubuntu's
default is to clear the screen, but I don't necessarily want to do
that, so I've just commented out the whole file. However, I added it to
the repo for reference, and as a reminder if we want to add future
functionality.
Add ~/.toprc to repo

This file holds our defaults for the `top` command (generated by
running top, setting its preferences to how you like, and writing
out these preferences by pressing 'W').

Presently, it shows mostly CPU usage, CPU Time, Mem %, process name,
username, and a bit more info, sorted by CPU usage.
Bash: fix rbenv support in ~/.bash_profile
Bash: add support for `keychain` utility

This utility manages running and queuerying the ssh-agent password
manager. We've aliased `ssh` and `git` so that running either commands
kicks off the keychain utility.