Added TwitterInputDStream from example to StreamingContext. Renamed example TwitterBasic to TwitterPopularTags.

As part of this, changed our Scala 2.9.2 Kafka library to be available
as a local Maven repository, following the example in
(http://blog.dub.podval.org/2010/01/maven-in-project-repository.html)

Added the io.spray JSON library

fix SequenceFileRDDFunctions to pick the right type conversion across Hadoop
versions

By default - I'm leaving this commented out. This is because
there is a bug in the PGP signing plugin which causes it to active
even duing a publish-local. So we'll just uncomment when we decide
to publish.

with Mesos being in Maven

This does a few things to get this branch ready for release:

1. Upgrades the sbt and Scala version
2. Sets the release number to 0.5.1
3. Adds the Sonatype publishing target
4. Installs the PGP signing plugin
5. Removes the Mesos jar dependency

the build file so that mesos-0.9.0-incubating.jar (which contains the
same class files, but has a silightly different name) will be pulled
down from Maven Central instead.

This reverts commit 42e0a680.