Bash: replace bash aliases for `brew` with wrapper

Previously, we were unable to directly alias homebrew commands, because
homebrew uses a single monolithic `brew` command to run all
sub-commands. So if we wanted to modify the `brew linkapps` command to
use the '--local' option by default, we would have to create a new
alias like `brew-linkapps` instead of being able to alias `brew linkapps` directly.

This change adds the `brew-wrapper` script which allows us to look at
which subcommand and options are being passed to the `brew` app, modify
them, and call into the `brew` app directly.

We then alias this script as `brew` so that it invisibly handles this
task.
Vim: change the color scheme to jellybeans+

This change affects both MacVim in GUI mode, and command-line Vim.
Vim: move PowerLine init from .vimrc to .gvimrc

Previously, in .vimrc, we were checking if the GUI was running, then
initializing PowerLine. This is silly; we can just put it in .gvimrc,
which is only read when in GUI mode. So we did.
Bash: remove some unused aliases (fxargs, fgrep)
Bash: rename 'fname' command to 'findfiles'

We also made it more robust: now, instead of always searching in the
current directory, it takes an optional second argument denoting the
search path.
Homebrew: automatically update before any upgrade