Added support for prototype cells in storyboard

[ifullgaz]

Added support for prototype cells in storyboard with a new example StoryboardCellTemplateExample.
Usage is simple:

1. Add a table to the form controller in storyboard and wire it to the tableView outlet
2. Add prototype cells as required
3. Assign a unique reuse identifier to each cell
4. In the form field definition add FXFormFieldCellIdentifier with value the cell identifier

[wm-j-ray]

Yay!

[petalvlad]

@nicklockwood what's your point about prototype cells? will it be supported?