Add support for datetime annotation types

[Nicklas Nordborg]

This is a rather big change and will have effect in other parts of BASE as well. Here is a list of things that we need to do.

• Add a new table for storing datetime values and a subclass to ParameterValues to handle it.
• Add a new option to the Type enumeration.
• Update the gui so that it is possible to add datetime values
• Update the query api so that it is possible to search for datetime values
• There are probably some effects also on the plug-in api since the Type enumeration and ParameterValues class are used by plug-in parameters as well. I'll have to investigate the effect and maybe add a specific ticket for adding datetime support to the plug-in system if the work required is substantial.
• There may be similar effects on "extendable" classes (eg. reporters, users and raw data).
• Check and update documentation
• More...?