Adding DateTime and Timespan support

[some1one]

…tion, and a dictionary of value getters can be used

[VitaliyMF]

In this pull request I see 2 different things.

1. They are should be separated as different PRs
2. regarding changes in LambdaParser.cs, I've provided my thoughts on this in Improve performance by compiling into lambda method calls, property getters etc #4. Variables caching is rather specific thing, and this is just several lines of code. It is a little sense to hardcode it into library.
3. I'm ok to accept changes in 'LambdaParameterWrapper.cs' related to TimeSpan / DateTime handling, however code should be improved:

Code like this has too strong assumption:

if (c1.Value is TimeSpan || c2.Value is TimeSpan)
{
	var c1TimeSpan = c1.Value as TimeSpan?;
	var c2TimeSpan = c2.Value as TimeSpan?;
	return new LambdaParameterWrapper(c1TimeSpan + c2TimeSpan, c1.Cmp);
}

What if c1.Value is DateTime, and c2.Value is TimeSpan? This is also correct set of arguments for data-type + or -, but your code will not work correctly.

My suggestion is to use strong type checks for + and - operations, smth like:

if (c1.Value is TimeSpan c1ts && c2.Value is TimeSpan c2ts)
	return new LambdaParameterWrapper(c1ts + c2ts, c1.Cmp);
if (c1.Value is DateTime c1dt && c2.Value is TimeSpan c2ts)
	return new LambdaParameterWrapper(c1dt.Add(c2ts), c1.Cmp);

Also, all this cases should be covered with tests.

If you don't want / don't have time to make all these improvements, just create an issue and I'll add support of TimeSpan / DateTime on occasion.

[some1one]

Okie dokie, I will remove the rest and just implement the date time stuff.

[some1one]

Alright, that should do it

[VitaliyMF]

Nice job, thanks!