[API] Added displaying the Elasticsearch build hash when running the YAML tests

$ rake test:integration

Will print out:

...
--------------------------------------------------------------------------------
                     Elasticsearch 0.90.7 [36897d0]
--------------------------------------------------------------------------------
...
[EXT] Removed storing the PID files in test cluster

We retrieve the PIDs from the "Nodes Info" API, not files.

Related: #16
[EXT] Added `es.path.data` and `es.path.work` to the test node configuration

Prevent accessing the default Elasticsearch data directory altogether,
default to /tmp, data path settable by method arguments or
the TEST_CLUSTER_DATA environment variable.

Closes: #16
[EXT] Allowed setting `gateway.type`, `index.store.type` and custom `-D es.*` params for the test cluster

TEST_CLUSTER_PARAMS='-D es.cluster.name=mycluster' rake test:server:start

Elasticsearch::Extensions::Test::Cluster.start \
  gateway_type: 'local',
  index_store:  'niofs'
[EXT] Added file with preconfigured Rake tasks for starting and stopping the test cluster