Date can be set at post creation

Added the parameter 'date' to the 'create_post' function.
The parameter is mapped to 'post_date' in Wordpress.
Post metadata can be set at post creation

Added the parameter 'meta' which contains an array of 'key' => 'value'.
The Wordpress function 'update_post_meta' is called for each pair.
Merge pull request #6 from fabienric/master

Post metadata can be set at post creation