Increase control over multiplexer auto-start

Setting the auto-start 'remote' and 'local' zstyles for the screen and
tmux modules to 'true' automatically starts a session in SSH connections
and in local terminals respectively.

Signed-off-by: Sorin Ionescu <sorin.ionescu@gmail.com>
Move from Carton to Cask

Carton was renamed to Cask to avoid a name clash with a Perl utility of
that name.
Clarify customization instructions

Explicitly state that a new user must create his own fork to commit and
track changes via Github.

Signed-off-by: Sorin Ionescu <sorin.ionescu@gmail.com>