Entering text should use native keyboard and placeholders

Steps defined in `calabash_steps.rb` do not match those described in [wiki](https://github.com/calabash/calabash-ios/wiki/02-Predefined-steps#wiki-entering-text). Also, we should use the native keyboard (since set_text is deprecated?) and fetch the text fields by placeholder (whereas in some places textfields were queried by `marked:`).