Setting version to 0.2.0:

The most significant change is updating to Titan `0.5.2`, which supports proper vertex types and a dramatically simplified type management interface. This introduced

- the `PropertyKey` used to model a vertex type is now `VertexLabelMaker`
- type creation is simpler and more expressive
- index creation can now be done through this API, without needing to resort to the raw Titan objects

Note that we use `xxxMakers` instead of types so that we can make this independent of any transaction management.

version.sbt

@@ -1 +1 @@
-version in ThisBuild := "0.2.0-SNAPSHOT"
+version in ThisBuild := "0.2.0"