Adding HBase raw tap

This change adds HBaseRawTap and HBaseRawScheme.

The point of this tap is to avoid the need of defining the input columns in the mapper, and allowing more control in handling hbase rows in a cascading (and scalding) job.

The source tap outputs pairs of (rowkey, row), where rowkey is the actual row object. So it's possible to collect and manipulate a changing set of columns in the mapper without predefining them. 
So, for instance, the first mapper in the pipe can transform the row like this (using scalding syntax) :

hbaseSource.map(('rowkey, 'row) -> ('key, 'field1, 'field2, 'field3)) 

where the output fields can be a combination of different columns in each row.

The source tap also adds support for providing a scan object (base64 encoded) for fully customizing the HBase read.

The sink tap expects a rowkey in the tuple, and will write other values as columns.