Fix up bash prompt and add git info

Fixed PS1 colour escape sequences so that bash knows the correct length
of the prompt. This fixes long commands overwriting the previous line in
the terminal.

Added git status info to the prompt via git-prompt.sh, which comes with
git.
Add music backup alias
Move ssh-agent init to after tmux

If an SSH agent is not required, pressing ctrl-c at the password prompt
leaves the terminal in a tmux session.