support jackson's fast number parsing/writing features

[pjfanning]

Jackson's JsonFactory supports these features.

• StreamReadFeature.USE_FAST_DOUBLE_PARSER
• StreamWriteFeature.USE_FAST_DOUBLE_WRITER

When you create a Jackson ObjectMapper, you can optionally supply a JsonFactory.

Just thought I'd raise awareness of these features of jackson. With the StreamReadFeature and StreamWriteFeature, you would probably want to make those opt-in - that json4s-jackson should probably not use them by default.