Adding support for ios8 cells

- In iOS8 to support self sizing cells we need to provide estimatedCellHeight, then set the `rowHeight` to be automatic
- to support `estimatedCellHeight` in formkit, we have variable estimates as specified by the cells themselves (default 44px)
- Once the cell has been displayed, we keep a cache of its actual height for future estimates, if this isnt done the tableview will jump when a cell is reloaded
- Bumped min version for FormKit to `7.1`