Add a preexec hook to set the GPG-Agent TTY

Signed-off-by: Sorin Ionescu <sorin.ionescu@gmail.com>
Check for S.gpg-agent to see if gpg-agent is running

From GnuPG changelog:

>  Removed the GPG_AGENT_INFO related code.  GnuPG does now
>  always use a fixed socket name in its home directory.

Signed-off-by: Sorin Ionescu <sorin.ionescu@gmail.com>
Fix issue with several themes where errors occur
if the git module is not loaded.
Fix Powerline unicode characters

Branch & separator characters are updated to display properly.