Performance tweaks

I played around with optimizing a couple of standard operations in streaming Python.  Here are the results:

1.  We no longer use blaze.utils.get inside inner loops in csv.  This cuts down the cost of queries like `dd[:, [field1, field2]]` by about an order of magnitude. 
2.  After this type coercion through DyND dominates runtimes.
3.  `SQL.extend` is faster about coercing tuples into dicts.  In a data migration the sqlite backend now dominates runtimes.
4.  I've also increased the size of the DyND block in `blaze.data.utils.coerce` to 1000.  This gave a 10% boost to a runtime, could also have been just noise.