Use a different character to delimit tags/values when encoded as a string

[tillberg]

@akahn pointed out that colons are legal characters in tag keys and values, and so we should use something other than "::" to delimit them when encoding as a string (in https://github.com/librato/appoptics-go/blob/master/tags.go), as a tag key or value that includes "::" would not be decoded correctly.

[trevrosen]

Alter to use a null byte literal \x00 as the separator