Remove JSX tag whitelist; use case instead

With this, tags that start with a lowercase letter are assumed to be built-in and have React.DOM prepended; uppercase tags are assumed to be user-defined and are left as-is.

Not sure if we want to do this but perhaps we do!
I need to test though it sounds like @sebmarkbage is moving some stuff around here anyway now that #2287 is in. We decided that tags with `-` and `:` are always treated as native tags (i.e., strings) and member expressions (with `.`) are always JS references, regardless of capitalization.