v2.0

@peterzakin @grgcombs What do you think about these changes for v2.0?

### `BZGFormFieldCell` -> `BZGTextFieldFormCell`
* Removes `voidEditingEventBlock` and `boolEditingEventBlock` types
  - autocomplete with inscrutable block types is annoying.
* Renames `showsCheckmark` -> `showsCheckmarkWhenValid`

### BZGFormViewController
* Renames:
  * `formFieldCells` -> `formCells`
  * `updateInfoCellBelowFormFieldCell` -> `updateInfoCellBelowFormCell`
  * `nextFormFieldCell` -> `nextFormCell`
  * `firstInvalidFormFieldCell` -> `firstInvalidFormCell`
* Removes `shouldShowInfoCell`
  - Use `BZGValidationStateWarning` when you want to show an info cell without marking the cell as invalid.

### Future changes
* Different cell types should inherit from a `BZGFormCell` abstract class.
  * e.g. `BZGFormCell` -> `BZGTextFormCell` -> `BZGTextViewFormCell`, `BZGTextFieldFormCell`