Create Ppl::Command::Execute

This is a command class whose sole purpose is to execute other commands in the
address book directory. This will be used for executing aliases beginning with
exclamation marks as well as for passing through certain subcommands straight to
git.
Sort the command list into alphabetical order in Ppl::Application::Bootstrap
Add support for execution of git's pull, push and remote commands via ppl

This allows users to run e.g. "ppl pull origin master" from any cwd. This allows
for fairly convenient usage of git to handle synchronisation without imposing
any tedious "cd" busywork upon users. Effectively these are pretty much
equivalent to these git aliases:

    pull = !git pull
    push = !git push
    remote = !git remote
Add descriptions to the three git command aliases
Prevent any optionparser interference in the execution of Ppl::Command::Execute

Optionparser was seeing any arguments to aliased commands as invalid usage.
Deciding things like that is the job of the executed command itself.
Version bump for new aliased git commands
Merge branch 'git'