Add mobile proxy

This is useful for using Java server commands directly.
On iOS, these commands are already accessible via execute_script.

Android example:
```ruby
mobile :proxy, command: :takeScreenshot
```
iOS example:
```ruby
mobile :proxy, command: 'au.capture("screenshot")'
```

On iOS, that's the same as
```ruby
execute_script  'au.capture("screenshot")'
```
--

If the code looks good, then I'll add some JavaScript tests to this pull request.