Add white option for slideshow shortcode.

As per @richardmtl in #30
Slideshow: White BG: Only override the color.

If someone has made modifications to the width, we don’t need to
specify those.
Don't bother with an is_admin() check before admin_init

If we’re not in the admin, adding this to admin_init won’t hurt
anything.
Remove needless callback.

Swap out `slideshow_section_callback` callback for
`__return_empty_string` for simplicity.
Whitespace tidy.
Pull the options array out and make it more readable.

Also, `__( ‘White’, ‘jetpack’, ‘jetpack’ )` had one too many
`’jetpack’`s
Swap order of conditions

Check whether $values is empty before checking if it’s not an array.
Whitespace tidying

Tidying up whitespace, and adding braces as per WP Core coding
standards.
Refactor `settings_select` for legibility.
Specify that the .slideshow-white is only on .slideshow-window element.
Tidy up how the variable for the color is popped back in.
Switch to using Jetpack_Options::get_option from get_option
Escape $extra_text on output.
Merge pull request #85 from Automattic/pr-30

Add white as an option for the slideshow background, see #30, #26