Setting version to 0.5.0:

Several breaking changes here, but mostly about names.

1. We dropped `Optional<Stream<X>>` in favor of just `Stream<X>`. An empty stream signals the equivalent of none before. See #40
2. We are using `E` as a _suffix_ for methods related with edges like `outE`, `inE`.
3. The names for arities have changed. See #41 and #40. They are defined in `TypedEdge.java`.

Apart from that, documentation should display better on github.com. The build plugin was also updated.

version.sbt

@@ -1 +1 @@
-version in ThisBuild := "0.5.0-SNAPSHOT"
+version in ThisBuild := "0.5.0"