Add build definition file for Circle CI

Configure the build for Circle CI in the new file circle.yml.
Specify a compile step to have the build fail on compilation error.
The test step is then configured as `ant test`, which will compile
again due to the limits of Ant/Moxie.

Contrary to the documentation, the default Java version on Circle CI
is Java 8.
The project is set as a Java 7 project. We define to use OpenJDK 7,
because the Gitblit build has some trouble with Java 8, I consider
Java 7 the default, and Circle CI does not provide an Oracle JDK 7
installation to use. I could only get it to work with OpenJDK 7.

The Java version is reported in the Circle CI build script to ease
analysis.

Test and coverage reports get stored as artifacts for a build, which
allows to browse them in the Circle CI web interface.

circle.yml

@@ -0,0 +1,22 @@
+machine:
+  java:
+    version: openjdk7
+
+compile:
+  pre:
+    - java -version
+    - javac -version
+  override:
+    - ant
+
+test:
+  override:
+    - ant test
+  post:
+    - mkdir -p $CIRCLE_TEST_REPORTS/junit/
+    - cp -a build/tests/TEST-*.xml $CIRCLE_TEST_REPORTS/junit/
+
+general:
+  artifacts:
+    - "build/target/reports"
+