Update 'rake post' to support category on command line

Updated comment.
Creating a post with multiple tags require no whitespaces between tags:
 rake post title="A Title" [date="2012-02-09"] ... tags=[tag1,tag2]
or quotes:
 rake post title="A Title" [date="2012-02-09"] ... tags="[tag1, tag2]"
Merge pull request #183 from Miguelos/patch-2

Update 'rake post' to support category on command line