Using sbt for building snappy-java

With this change, there are several improvements:
 * No need exists to install maven to build snappy-java. The bundled sbt script automatically downloads necessary jars. 
 * We can remove lib/silk-weaver.jar, which is used to parse xml files. Now Makefile uses a simple perl script to obtain `version` number, written in `version.sbt`
 * With sbt-sonatype plugin, releasing to the Maven central can be much easier and faster. 
 * Running tests in parallel is possible with sbt