function to export tables from teradata to a file

This is a similar function to tdBigQuery() but instead of keeping the file in memory, it exports 
the file in pieces and appends it to a file as sometimes this is useful and fast export is more complicated to use.
Probably the write.table arguments could be added to the function arguments.
Merge pull request #2 from martinbel/patch-1

function to export tables from teradata to a file