Add support for phantom's #renderBase64.

The options for `#render_base64` and `#render` are the same. They are aliases
of `#save_screenshot` and `#save_screenshot_base64` respectively. This closes
#189.

Usage:

#### Current Viewport
```ruby
image = @driver.render_base64
some_io.write image
```

#### Entire Window
```ruby
image = @driver.render_base64 full: true
some_io.write image
```

#### Selector
```ruby
```