Issue #158: org.apache.thrift#libthrift;0.5.0 doesn't exist

1. All submodules of the finagle parent POM should reference that parent POM (rather than the `scala-parent-292` artifact).
2. The finagle parent POM should have `scala-parent-292` as its parent.
3. The finagle parent POM should include a reference to Twitter's public maven repo, so that finagle users can resolve transitive dependencies (such as `org.apache.thrift#libthrift;0.5.0`).