Added new TimeSpan type and XOR operator

[brianhassel]

Great project; thank you!

To understand it better, I added an XOR operator and a TimeSpan type. The TimeSpan is parsed like a typical System.TimeSpan wrapped in tick (ASCII 96) characters:

`1.03:00:00`

I changed the DateTime type to return a TimeSpan when subtracting 2 DateTimes, and a TS can be subtracted/added to a DT.

I'll also pull request my unit tests.

[hisystems]

Hey Brian,

Thanks for the pull request. It would be great to support Timespan as construct. So thanks for that. However, there are few things with the changes. Can you;

• Use spaces / rather than tabs?
• Separate out the XOR work from the Timespan work into different commits?
• I currently need to support an expression such as #1/1/2000# - 1 (where 1 is interpreted in days). This is used in one of my projects and it looks like that wouldn't work. Can we manage backward compatibility somehow?