Add Android dynamic find

Fix #387

Appium iOS allows sending a custom JavaScript program that can search for anything in one server call. In calabash-android there's a query language that serves a similar purpose. Appium Android currently doesn't have anything like that.

My use case is being able to have a version of [this generic find](https://github.com/appium/ruby_lib/blob/master/lib/appium_lib/element/ios/generic.rb#L23) method for Appium Android.

Here's a working Ruby example.
```ruby
> mobile :dynamic, [[3, 'sign']]
post
session/0d85eb7d-74a4-49bd-9215-306f2557d911/execute
{"script":"mobile: dynamic","args":[[[3,"sign"]]]}
=> [#<Selenium::WebDriver::Element:0x752ef7d0a9fd80a4 id="4">]
```