YAML marshaling and unmarshaling support for Go

This is a fork of github.com/ghodss/yaml to provide modified YAML and JSON parsing for use with libcalico-go and calicoctl. The modifications include:

Swapping support of Float32 with Float64 (since calico does not use Float32)
Providing the ability to perform strict unmarshaling (i.e. erroring if a field in the document was not in the struct)
Slightly modified error messages to be less go-lang oriented and more user facing.
Indication of field names or values