More reliable concurrent schema modifications

Cassandra 2.0 and 2.1 does not like concurrent schema changes (see
https://issues.apache.org/jira/browse/CASSANDRA-8387). Until there is a better
solution in Cassandra itself, make our table creation more reliable by
retrying it a limited number of times. This is safe, as our table creation
code is idempotent.
Release restbase-cassandra v0.3.0
Merge pull request #29 from gwicke/master

Fix table creation issues by retrying & release v0.3.0