Use underlying float precision when formatting floats (go-yaml#353)

Currently, all float values are formatted using 64-bit, but that
incorrectly formats float32 values like 0.01 and 0.99.

See https://play.golang.org/p/jbseI1ivyMW for more context.

Fixes go-yaml#352.

Drop unnecessary explicit timestamp tags.

Fixes go-yaml#341.

tag v2.1.1

tag release 2.1.0

tag v2