proper escaping of backslashes

fixes https://github.com/calabash/calabash-ios/issues/284

## Known Issues

none

## Notes

There are some interesting differences in the cucumber output.

```
# without Instruments on devices
When I type a the following string with a backslash "This string \ has a backslash

# with Instruments on devices
When I type a the following string with a backslash "This string \\ has a backslash
```

## Testing

### Briar

* [briar 0.1.3b7](https://github.com/jmoody/briar/tree/0.1.3b7)
* [briar-ios-example 1.1.5](https://github.com/jmoody/briar-ios-example/tree/1.1.5-morekeyboard)
* tags 
    - @issue_284 - just backslash testing
    - @keyboard - all keyboard tests
* ruby versions
   - [x] ruby 2.0
   - [x] ruby 1.9
   - [x] ruby 1.8 
    
#### `NO_LAUNCH=1` ==> Manually Launching ==> UIA _not_ available

- [x] iPhone 4S iOS 6
- [x] iPad 1 iOS 5.1.1 
- [x] iPhone Simulator iOS 6.1
- [x] iPad Simulator iOS 6.1

####  `NO_LAUNCH=0` ==> Launching with Instruments ==> UIA available

- [x] iPhone 4S iOS 6 
- [x] iPhone 5C iOS 7.0.2
- [x] iPad 4 iOS 7.0.3
- [x] iPhone Simulator iOS 7
- [x] iPad Simulator iOS 7

#### `NO_LAUNCH=0` ==> Launching with Sim Launcher ==> UIA _not_ available

- [x] iPhone Simulator iOS 6.1
- [x] iPad Simulator iOS 6.1

### XTC

### LPSimpleExample (smoke test)
[LPSimpleExample](https://github.com/jmoody/calabash-ios-example)

- Apple iPhone 5C (7.0.4)
- Apple iPhone 4S (6.1.3)
- Apple iPad 4th Gen (7.0.4)
- Apple iPad 2 (5.1.1)

- [x] ruby 2.0
- [x] ruby 1.9 
- [x] ruby 1.8 - had to specify gem 'mime-types', '1.25'

### Briar iOS Example @issue_284

[briar-ios-example 1.1.5](https://github.com/jmoody/briar-ios-example/tree/1.1.5-morekeyboard)

- [x] Apple iPad 2 (5.1.1)
- [x] Apple iPad Air (7.0.4)
- [x] Apple iPhone 4S (5.1.1)
- [x] Apple iPhone 4S (7.0.4)
- [x] Apple iPhone 4S (6.1.3)
- [x] Apple iPhone 3GS (6.1.3)
- [x] Apple iPad 2 (6.1.3)
- [x] Apple iPad 4th Gen (7.0.4)
- [x] Apple iPhone 5 (6.1.4)
- [x] Apple iPhone 5 (7.0.4)
- [ ] Apple iPhone 5C (7.0.4) - 'VerboseError: target.frontMostApp().keyboard() failed to tap 
- [x] Apple iPhone 5S (7.0.4)
- [x] Apple iPod touch 5th Gen (7.0.4)
- [x] Apple iPod touch 5th Gen (6.1.3)