Build exporters with maven. (#76)

Author: Bogdan Drutu

pom.xml

@@ -52,6 +52,7 @@
           <pluginArtifact>io.grpc:protoc-gen-grpc-java:${grpc.version}:exe:${os.detected.classifier}</pluginArtifact>
           <protoSourceRoot>.</protoSourceRoot>
 	        <includes>
+            <include>opencensus/proto/exporter/*.proto</include>
             <include>opencensus/proto/stats/*.proto</include>
             <include>opencensus/proto/metrics/*.proto</include>
             <include>opencensus/proto/trace/*.proto</include>