Fix: Make the JNI interface actually work

Now a user can import LTTngUst.java (which is now in a proper
package) and then define tracepoints in his application with:

LTTngUst.tracepointString(name, args);

We could eventually ship it in a .jar when the JNI interface is
enabled. It's still very basic, only string payloads are
implemented, and there is no mechanism to enable/disable trace
points call sites at runtime (they are always enabled). The event under
which these events appear is "lttng_ust_java:string".

Signed-off-by: Alexandre Montplaisir <alexandre.montplaisir@polymtl.ca>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>