issue 151: general clean up of text field input and disabled keyboard_enter_text for iOS 7

### POTENTIAL BREAKING CHANGES: the predefined steps for text fields:

1. no longer call set_text - they call keyboard_enter_text
2. they first try to touch the text field indicated by the arguments
   and wait for the keyboard before they try to enter text

### IMPORTANT

`keyboard_enter_text` now throws a `pending` exception on iOS 7 because using it on that OS causes the app to enter an infinite loop

### Motivation

https://github.com/calabash/calabash-ios/issues/151

the `set_text` function is marked as deprecated, but it is still used in many predefined steps.  the motivation behind the deprecation is sound:  we want people to migrate to `keyboard_enter_text`.  however, there are still times when you want to clear the text from a text field or text view.  to meet this need, i added `clear_text`.  

since we want people to stop using `set_text`, i rewrote the predefined steps to use `keyboard_enter_text` and `clear_text`.  the predefined steps will also touch now touch the correct text field and wait for the keyboard, which will obviate the need to write additional steps like:

```
Then I touch the "password" text field
# for keyboard animation
Then I wait 
```
before a step like:

```
Then I enter "pa$$w0rd" into text field number 2
```

tested on briar-ios-example 0.0.8 - https://github.com/jmoody/briar-ios-example/tree/0.0.8