Shutdown cluster in migrate & initialize

This should be done to release resources, which is relevant when used
in a running application (e.g. Play).

Another great thing to accept an existing session in CassandraMigrator,
so that the application using it could just pass the session its
creating anyways. I didn't do this because migrate/initialize right
now accept a DataStore which would then be obsolete, at least it's not
straight forward to allow both - probably there would be 2 constructors:
1 accepting a DataStore and another one accepting a session.
Change cluster.shutdown to cluster.close (after update to datastax 2)
Merge pull request #5 from magro/shutdown-cluster

Shutdown cluster in migrate & initialize
Remove Gradle build, add empty build.sbt so that IDEA can import project
Clean up ignored files
Rebrand from StreamSend to Chris O'Meara
Add migrate/initialize/destroy accepting a Session.

This is useful for apps embedding pillar as a library, so that they can
provide their already existing session.

Adds library acceptance specs for migrator+session extensions, while
not each scenario is duplicated for the session interface, for each
feature it's just one scenario testing the interface taking a session.

String.format is replaced with string interpolation as it's more readable.