Function to scan an HBase table and return a data frame.

The row names of the resulting data frame are the keys of the HBase table. The column names are those specified in colspec.  If a row does not have a value for a column specified in colspec, then that cell will
be populated with NA.

Discussion here: https://groups.google.com/d/msg/rhadoop/Oty-A0UodTM/ngbLVAFRX0oJ