remove unused variables
use registermultitask

Use
[grunt.task.registermultitask](http://gruntjs.com/api/grunt.task#grunt.task.registermultitask)
in favor of grunt.task.registertask. This allows for setting up multiple
targets (dev vs continuous) and sharing common options through the
options property.
switch merge for extend

Otherwise array properties (like specFolders) will be merged instead of
replaced.
Updated readme for registermultitask