sql <- csv with gzip gracefully falls back to other method

Conflicts:
	blaze/data/sql.py
remove cautious kwarg checking

is this ok @quasiben?
count_values is usable by non-columns
into DataDescriptor < ChunkIterator just like mongodb
csv-gzip-sql test uses binary write mode
Merge pull request #709 from mrocklin/various-fixes-3

Various fixes 3