Bash: set LC_CTYPE=en_US.UTF-8 for tmux UTF-8

This change is specifically aimed at allowing tmux to work with
Powerline.

We already have LANG=C in ~/.bash_profile, which is what we want most
of the time (allows the output of `ls` to sort dotfiles first, etc.)

However, this caused tmux to believe we weren't running in a UTF-8
environment. This caused problems because Powerline uses UTF-8
characters as dividers (arrows and the like.) Setting options in
tmux.conf to explicitly enable UTF-8 did not seem to work (I suspect
that it read Powerline's tmux.conf before those options took effect,
and it would be confused by the UTF-8 characters in the conf file.)

One way to fix this was to always start tmux via `tmux -u`, which would
force UTF-8 mode on. However, this was undesirable for obvious reasons
(even if we aliased `tmux` to `tmux -u`, we may not catch tmux being
launched by daemons, etc.)

By setting Bash's LC_CTYPE to a UTF-8 character set (en_US.UTF-8), tmux
(which looks at the values of LANG, LC_CTYPE and LC_ALL to figure out
if it's running in an UTF-8 environment), tmux will always start in
UTF-8 mode without launching it with '-u'. Further, by leaving LANG=C,
we retain those desirable features (proper `ls` sorting, etc.) without
any other side effects (LC_CTYPE, if I am reading right, only sets theg
character type Bash translates output to.)
Bash: remove unused $color_prompt var from profile

As a vestige of the original Ubuntu ~/.profile, we detected the $TERM
type and then set the variable $color_prompt if we thought the term
supported colors.

The original Ubuntu .profile then took some later actions based off if
$color_prompt was set (like enabling a color $PS1.) However, in our
~/.bash_profile, we assume all terminals have color (because, come on,
who doesn't???) We therefore never actually used the $color_prompt
variable.

Accordingly, we remove the test and setting of $color_prompt for
~/.bash_profile.
Bash: tweak $PS1 if running in tmux

We tweak the way we set our $PS1 and $PS2 in ~/.bash_profile to support
using slightly different settings in tmux.

We first refactor the setting of $PS1 to extract the shell level
variable from being used directly in the $PS1 string (as $SHLVL), into
the variable $NUM_PROMPTS, which we set before setting $PS1, and use in
$PS1 in place of $SHLVL.

We then add code before we set $PS1 to detect if $TERM is tmux. If so,
we set $PROMPT_COLOR to be a slightly different color (same bg color,
but a light yellow fg color.)

If the $TERM is tmux, we also subtract 1 from $NUM_PROMPTS. This is
because tmux itself adds another level to $SHLVL, but seeing this
doesn't really tell us anything useful (that knowing we're running in
tmux, via the different prompt color and Powerline status line) doesn't
tell us.
Top: tweak .toprc

We tweak the default colors slightly, and use the low-resolution (seconds,
not fractional seconds) CPU TIME field.