Using sbt for building snappy-java

[xerial]

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