updated the deprecation warning for set_text
BREAKING CHANGES: general clean up of text field interaction around set_text

* keyboard_helpers: IMPORTANT - disabled keyboard_set_text for iOS 7
  because it throws the app into an infinite loop
* operations: added clear_text function and improved deprecated
  message for set_text
* calabash_steps: migrated predefined steps away from set_text to
  keyboard_enter_text and (where applicable) to clear_text.  added a
  deprecation warning to 'When I clear' step because it is an
  ambiguous step - what am i clearing?

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

tested all text field related predefined steps with briar-ios-example
0.0.8 on iOS 5 - 7 on iPhone and for iOS 5 - 6 on iPhone app emulated
in iPad.

https://github.com/calabash/calabash-ios/issues/151
Merge pull request #183 from jmoody/0.9.x-issue-151-clarify-set_text-deprecation-warning

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