…ags to have the specified author/email

Support reading GIT_AUTHOR_{NAME,EMAIL} env variables

Change the discovery of the user.name and user.email variables to
first look up the user/name in the GIT_AUTHOR_NAME and
GIT_AUTHOR_EMAIL environment variables. This mirrors the behavior of
Git itself, see git-commit-tree(1) for details.

The reason to do this is that managing your Git identity via these
variables can be more conveniente than setting them in a config file,
it makes it easier to set them differently on different hosts.

Also change the error message we display if these aren't set, we'll
now error out with something like this when we can't get an author
name or an E-Mail;

    $ perl -Ilib bin/git-deploy start
    # FATAL: Please make sure you set your Git user name & E-Mail
    # FATAL: before using this tool, i.e. either run:
    # FATAL:
    # FATAL:     git config --global user.name 'Your Name'
    # FATAL:     git config --global user.email 'email@host.com'
    # FATAL:
    # FATAL: Or make sure the GIT_AUTHOR_NAME and GIT_AUTHOR_EMAIL
    # FATAL: environment variables are appropriately set.

This patch is inspired by Rob's
git-deploy#20, I thought it made
more sense to introduce a function for looking up the user name &
E-Mail rather than hooking into the low-level config mechanism. This
also adjusts our documentation to indicate to users that they can use
this instead of setting user.{name,email}.