Force ssh-agent output to use bourne-style syntax

On systems where the shell cannot be changed because of a strict
security policy, ssh-agent will use the syntax of whatever the
default $SHELL is.
For instance, if the default shell is tcsh, ssh-agent will use the
c-shell style (setenv).

This change forces ssh-agent to use bourne-style syntax since that
has to be later interpreted by zsh. Consequently, the environment
file will contain `export' statements from now.
Include batcharge.py into batcharge function

This prevents a not found error when the batcharge.py script cannot be
found, which happens whenever we checkout a point in time where the
batcharge plugin is not there.

By including it directly into the batcharge plugin function we achieve
both the disappearance of the error mentioned above and ensure a fully
functioning batcharge plugin (as long as the batcharge function is
still there).

An alternative solution was to put a check for the batcharge.py script
inside the batcharge function, before calling it with python. This,
however, wouldn't mantain the plugin's functionality.

A desirable future change will be to translate the python script into
zsh code, so as to avoid running a separate process in every call of
the batcharge function and remove the necessity of a python binary.
Merge branch 'ssh-agent-force-bourne-syntax' into dev