[BUG] Error: ExpressionChangedAfterItHasBeenCheckedError

**Problem**
I have a `valuePrepareFunction` that is calculating time from now using a momentjs method. If I load the table and a cell has a value of "a minute ago", then after 1 minute passes, the view value updates to "two minutes ago". Then when I try to sort/filter the error: ExpressionChangedAfterItHasBeenCheckedError is thrown and the table freezes. User can no longer interact with the table.

I believe this template is the culprit: https://github.com/akveo/ng2-smart-table/blob/master/src/ng2-smart-table/components/cell/cell-view-mode/view-cell.component.ts.

I think the error occurs because `cell.getValue()` is directly bound to in the view & changes due to the `valuePrepareFunction`. There should be a way to handle this updated view value gracefully & avoid having the table lock up/freeze. I'm still digging into this issue. Will report back with any interesting findings. Or will submit PR.

Links to information about the error:
https://github.com/angular/angular/issues/14748
https://github.com/angular/angular/issues/15464
https://stackoverflow.com/questions/39787038/how-to-manage-angular2-expression-has-changed-after-it-was-checked-exception-w