jmxtrans-agent-1.2.0

• Add LibratoWriter thanks to @nbvehrfr

• Add support for dynamic agent attachment thanks to @pingtimeout

  With an "agentmain" method, it is possible to attach the agent to a
  running JVM. This PR adds a new class that connects to an existing JVM
  and attach the agent after the fact.
  To use the agent, the absolute paths to both the agent JAR and its
  configuration must be passed to the launcher, along with the target JVM
  PID.

  Example, assuming the agent is in /tmp/, the configuration file is
  /tmp/sample-config.xml and the target JVM has PID 1234, run the following:

  java \
  -cp $JAVA_HOME/lib/tools.jar:/tmp/jmxtrans-agent-1.2.0.jar \
  org.jmxtrans.agent.DynamicallyAgentAttacher \
  /path/to/jmxtrans-agent-1.2.0.jar \
  1234 \
  /path/to/jmxtrans-agent-config.xml