This commit is a fix for [bug #28](https://github.com/SchapplM/xbmc-a…

[fabquenneville]

Hello

This commit is a fix for bug #28

• Changed the name of the utils method TimeStamptosqlDateTime to TimeStamptostringDateTime as SQL would not allow an empty string time and in the current usage it ends converted to '1970-01-01 00:00:00' when combined with a query using FROM_UNIXTIME. Also this function was not used in SQL context but in logging context.
• Changed all references to TimeStamptosqlDateTime with TimeStamptostringDateTime to reflect the point above.
• Removed the usage of FROM_UNIXTIME and replaced that with a utils method
• Recycled the above method name TimeStamptosqlDateTime to create a new method returning a valid datetime or the object None instead of emptyness to be used in SQL context as mysql.connector then converts that to NULL when executed.
• Filtered all the old execute values that used to use FROM_UNIXTIME to now go trough TimeStamptosqlDateTime instead to correct errors at the add-on level instead of in the query.

[fabquenneville]

The fix has created another bug. I think it is simple to fix I will re-request later.