Update the help for deactivate
Properly exit from activate or deactivate if called without source
activate and deactivate change the PS1 (in Unix)

This fixes #145 (but not yet on Windows).  By default, it just prepends
($CONDA_DEFAULT_ENV) to the PS1. You can disable this by putting

changeps1: no

in your .condarc.  You can create your own custom PS1 by using the
$CONDA_DEFAULT_ENV environment variable.