Fixed a typo in smptett header
Fixed smptett timeExpression format

Now all timeExperssion in smptett file comply to:
http://www.w3.org/TR/2013/REC-ttml1-20130924/#timing-value-timeExpression
Added support for -startat and -endat for teletext input
Fixed potential timestamp overflow with negative delay

When delay was a large enough negative number, some show_timestamp would
overflow and be set to ridiculously large value.