Add build() overload that doesn't use Environment

Sometimes, you will want to configure and build a `Cluster` instance
using `CassandraFactory`, but you won't have an `Environment` available.

The common-case here is when writing a `ConfiguredCommand` that uses
Cassandra (e.g. for a utility to execute your applications' DDL
statements).

While we have no `Environment`, we can still register health-checks and
metrics, but we cannot manage the Cluster lifecycle, as there's no
Environment to tie its lifecycle to.

This is broadly following the idiom used by the Liquibase Commands in
`dropwizard-migrations` and the `DataSourceFactory` it uses to configure
connections.
Changed CassandraHealthCheck to use a validationQuery instead of Host.isUp

It seems the Cassandra driver is far too slow to determine host availability for the purposes of a health check, leading to false positives. This has been replaced with a validation query (with a default value) that is executed against the cluster. This is very reliable as the query only succeeds if it can be executed against a node in the cluster.
Updated docs for CassandraHealthCheck changes
Add JavaDoc to CassandraFactory#build overloads
Merged changes from PR6
Merge branch 'nicktelford-feature/factory-no-environment-overload'

This merge is from PR6 and includes the ability to build a cluster without an Environment.
Updated CHANGES
[maven-release-plugin] prepare release v0.6
[maven-release-plugin] prepare for next development iteration
Updated CHANGES for 0.6 release
Upgraded to 2.0.3 Datastax Cassandra Driver
Merge pull request #7 from manub/master

Upgraded to 2.0.3 Datastax Cassandra Driver
Added PR7 to CHANGES. Changed PoolingOptionsFactoryTest values to be unique.
[maven-release-plugin] prepare release v0.6.1
[maven-release-plugin] prepare for next development iteration
Updated CHANGES
Updated plugin versions. Updated project versioning to incorporate Dropwizard version
Updated CHANGES