Pass the directory from which grunt was run to the hook template

The directory is passed as 'gruntfileDirectory' so the template can
use it to change the directory before running grunt. This is important
if the .git folder is not in the same directory as grunt.
The default templates use the variable 'gruntfileDirectory' to change
the current directory before executing grunt.
Updated tests to account for the addition of the path to the file containing Gruntfile.js in generated hooks. Also made sure the use of this directory as working directory only impacts the part of the hook running grunt, in case hooks are inserted in existing git hooks with code dependent on working directory
Updated documentation