adds support for manipulating UIDatePickers with LPDatePickerOperation

#### additions

adds a new function `picker_set_date_time(date_time, opt)` or interacting with LPDatePickerOperation

adds three predefined steps to demonstration how to use `picker_set_date_time`

```
# pass any string that can be parsed by Time.parse
Then I change the date picker time to "10:45"
# pass any string that can be parsed by Date.parse
Then I change the date picker date to "July 28 2009"
# ditto
Then I change the date picker date to "July 28" at "15:23"
```
#### KNOWN ISSUES

does not support pickers in count-down mode (yet)

#### tested

* [briar-ios-example](https://github.com/jmoody/briar-ios-example)  {master | 0.0.9}
* [briar](https://github.com/jmoody/briar) {master | 0.0.9}
* iOS 5 + 6
* simulators: iPad + iPhone 
* devices:  iPad 1 (iOS 5) + iPhone 4S (iOS 6)

* `$ cucumber --tags @calabash_date_picker`
* `$ cucumber --tags @date_picker`