Add Script tag with AMF3 content.

This doesn't parse the tag, but it at least recognizes it in the stream
and continues on. Wikipedia calls this the RTMP Stream Flex Send tag
while the RTMP docs call it a data tag containing AMF3.
Cleanup ScriptAMF3Tag

Add some simple tests and address concerns from review:
- AMF3 v. Amf3
- Unnecessary constructor override
- tag_to_class ordering
Merge pull request #1 from byronclark/additional-flv-tags

Add Script tag with AMF3 content.