Creation of executable jars with sbt-assembly

Allows creation of executable jars via sbt in addition to Maven (see https://github.com/factorie/factorie/issues/40). To make this work, I have added the sbt-assembly plugin and updated the sbt launch script to work with sbt 0.13 (I simply copy-pasted from https://github.com/paulp/sbt-extras/blob/master/sbt). README updated with instructions for running with sbt.