Add support for fetching an individual lazy property with Criteria

[bahusoid]

Follow up to #1922
To avoid accidental misuse with SelectMode.FetchLazyProperties new enum is called SelectMode.FetchLazyPropertyGroup (and not SelectMode.FetchLazyProperty)

Example of usage:

var entity = session.QueryOver<Entity>()
	.Fetch(
		SelectMode.FetchLazyPropertyGroup,
		 ec => ec.LazyProp1, ec => ec.LazyProp2, ec => ec.OtherEntity.LazyProp3)
	.Where(ec => ec.Id == id)
	.SingleOrDefault();

Fixes #1861

[hazzik]

To avoid accidental misuse with SelectMode.FetchLazyProperties new enum is called SelectMode.FetchProperty (and not SelectMode.FetchLazyProperty)

What about changing name of the new value to FetchLazyPropertyGroup and old one to FetchAllLazyProperties (added "all")?

[hazzik]

The commit with proposed changes is here: 6f38e03.

[bahusoid]

FetchLazyPropertyGroup looks good and it makes it visually distinguishable with FetchLazyProperties. Renaming seems unnecessary to me. But no objections.