Implemented function parseDateString

git-svn-id: https://plugins.svn.wordpress.org/wp-simile-timeline/trunk@996730 b8457f37-d9ea-0310-8a92-e5e31aec5664
Prepared saving of custom date string with prefix

git-svn-id: https://plugins.svn.wordpress.org/wp-simile-timeline/trunk@996733 b8457f37-d9ea-0310-8a92-e5e31aec5664