Add support for phantom's #renderBase64.

The options for `#render_base64` and `#render` are the same except the first
argument which is format for `#render_base64`. Closes #189

Usage:

    encoded_image = @driver.render_base64(:jpeg) # => :png by default
    some_io.write image

    encoded_image = @driver.render_base64 :png, :full => true
    some_io.write image

    encoded_image = @driver.render_base64 :png, :selector => '.class'
    some_io.write image