Fixed invalid array key for start- and end_date

git-svn-id: https://plugins.svn.wordpress.org/wp-simile-timeline/trunk@993931 b8457f37-d9ea-0310-8a92-e5e31aec5664
Table installation still with warnings. TODO: Check for duplicate columns

git-svn-id: https://plugins.svn.wordpress.org/wp-simile-timeline/trunk@993933 b8457f37-d9ea-0310-8a92-e5e31aec5664