Have [switch] match uppercase

Since WordPress converts all shortcode attributes to lowercase during
processing, our shortcode will never match uppercase letters in either
the $_GET or $_POST globals.

After checking that the field exists, it needs to be run through
`strtolower()` to have at least a “Case Insensitive” match.

This issue should be revisited later to make the match
“Case Sensitive,” but that may require modification to WordPress Core.

[Addresses: GH-12]
Merge pull request #12 from gMagicScott/switch-match-upper

Switch shortcodes should match uppercase variables