Add and use BitMaskStringBuilder

[JakeWharton]

public class BitMaskStringBuilder
  private final int flags;
  private final Set<String> parts = new LinkedHashSet<String>();

  public BitMaskStringBuilder(int flags) {
    this.flags = flags;
  }

  public BitMaskStringBuilder flag(int flag, String flagName) {
    if ((flags & flag) != 0) {
      parts.add(flagName);
    }
  }

  public String get() {
    StringBuilder result = new StringBuilder();
    for (String part : parts) {
      if (result.length() > 0) {
        result.add(", ");
      }
      result.add(part);
    }
    return result.toString();
  }
}

[edenman]

Starting on this now.

[edenman]

Fixed by #54