Fix issue: scan operation  and HBaseInterface closed.

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<li>
There maybe an issue in the scan operation because the HBaseInterface was closed before retrieving the information. It was reported <a href="https://github.com/mulesoft/hbase-connector/issues/2"> here </a>.

The proposed solution avoid closing the HbaseInterface instance in case the scan method is invoked. That way the data could be retrieved once it is needed. This instance will be closed once there is no more data retrieved in the ScannerAndResults implementation.
</li>

<li>
The "check and delete" operation allows passing null as value. According to the <a href="http://hbase.apache.org/0.94/apidocs/org/apache/hadoop/hbase/client/HTableInterface.html#checkAndDelete(byte[], byte[], byte[], byte[], org.apache.hadoop.hbase.client.Delete)">Hbase API </a>, this is allowed in order to check for the lack of a column.
</li>
<li>
Changed the scope of createHTable method in order to make easier to extend the RPCHBaseService.
</li>
</ul>