ForKind should return an error instead of panicing

[Lukas Malkmus]

The ForKind function panics for an unknown column type:

column/column.go

Line 115 in 1f327b4

 panic(fmt.Errorf("column: unsupported column kind (%v)", kind)) 

But it would be nicer if it would error and be picked up by:

column/collection.go

Line 230 in 1f327b4

 c.CreateColumn(k, ForKind(reflect.TypeOf(v).Kind())) 

[Roman Atachiants]

Added in the last PR #39