changes the text field touch predefined step to search for text fields by 'placeholder' and by 'marked'

it is a  simple change and is backward compatible.

can be tested using:

https://github.com/jmoody/briar/tree/0.0.6
https://github.com/jmoody/briar-ios-example/tree/0.0.6

the relevant feature file in the ios example is:   Briar/features/calabash_pull_request_116.feature

test is currently failing pending the change