Basic april tag support

Basic java-based april tag detection (at this time hardcoded for family 36h11), and a message publishing node.

April tag detection uses `java.awt.BufferedImage` (unavailable in android), as a temporary workaround a prebuilt jar is provided, to build it apply the following patch on the april tag sources

    diff --git a/java/src/april/tag/TagDetector.java b/java/src/april/tag/TagDetector.java
    index 0a8dd01..d20f5e7 100644
    --- a/java/src/april/tag/TagDetector.java
    +++ b/java/src/april/tag/TagDetector.java
    @@ -244,6 +244,12 @@ public class TagDetector implements AbstractTagDetector
          **/
         public ArrayList<TagDetection> process(BufferedImage im, double opticalCenter[])
         {
    +        return processFloat(new FloatImage(im), opticalCenter);
    +    }
    +
    +    /** Detect the features in the specified float image.  **/
    +    public ArrayList<TagDetection> processFloat(FloatImage fimOrig, double opticalCenter[])
    +    {
             this.opticalCenter = opticalCenter;
 
             // This is a very long function, but it can't really be
    @@ -253,8 +259,6 @@ public class TagDetector implements AbstractTagDetector
             ///////////////////////////////////////////////////////////
             // Step one. Preprocess image (convert to float (grayscale)
             // and low pass if necessary.)
    -        FloatImage fimOrig = new FloatImage(im);
    -
             FloatImage fim = fimOrig;
             if (sigma > 0) {
                 int filtsz = ((int) Math.max(3, 3*sigma)) | 1;