Added padding for requested columns not found in file schema

This change allows specification of columns within a requested schema that do not exist within the file.  Those columns will then result in null fields.  This is to facilitate schema migration using an external metastore as the source for schema information.

Example:
File Schema:  c1, c2, c3
Table Schema: c1, c2, c3, c4

Prior to this patch, if the table schema was supplied to the parquet loader, it would result in a tuple mismatch in pig.  Now it will pad the missing column will an empty tuple value.  This also works for rearranging the columns (e.g. c1, c4, c2, c3) and will pad the correct column position.