implement "b3 single" header format

[codefromthecrypt]

As discussed on openzipkin/b3-propagation#21 and first implemented here: https://github.com/openzipkin/brave/blob/master/brave/src/main/java/brave/propagation/B3SingleFormat.java https://github.com/openzipkin/brave/blob/master/brave/src/test/java/brave/propagation/B3SingleFormatTest.java

Let's support at least reading "b3" header from a single string, most commonly traceid-spanid-1
It would also be nice to support optionally writing this, especially in message providers or others with constrained environments.

Brave currently has a property like this, but its name could change with feedback:

    /**
     * When true, only writes a single {@link B3SingleFormat b3 header} for outbound propagation.
     *
     * <p>Use this to reduce overhead. Note: normal {@link Tracing#propagation()} is used to parse
     * incoming headers. The implementation must be able to read "b3" headers.
     */
    public Builder b3SingleFormat(boolean b3SingleFormat) {
      this.b3SingleFormat = b3SingleFormat;
      return this;
}

[jcchavezs]

Closed by #151