Add messages to SystemMetricsActor to retrieve memory usage.

Use MemoryMXBean to to retrieve heap and non-heap memory usage from the JMX
agent.
Add sigar to SBT library dependencies.

sigar provides system monitoring data such as:
- System memory, swap, cpu, load average, uptime, logins
- Per-process memory, cpu, credential info, state, arguments, environment, open files
- File system detection and metrics
- Network interface detection, configuration information and metrics
- Network route and connection tables

- Native classifier is used because sigar includes native libs
- copyNativeLibraries task is added to unzip sigar native libraries under target/native_libraries/

See
http://stackoverflow.com/questions/14966414/including-hyperic-sigar-library-within-jar-while-using-sbt-assembly-for-scala-pr
http://stackoverflow.com/questions/19303553/adding-libsigar-shared-library-in-sbt-project-without-using-java-library-path?rq=1
Add messages to SystemMetricsActor to retrieve swap memory usage.

Use SigarSwap MBean to retrieve swap memory usage.