ENH: add chunksize argument to read_sql (GH2908)
Closes #2908 

Should probably add this also to `read_sql_table`. And try to reduce some repetitions (`_wrap_result` can maybe be a top-level (in the file) helper function).

@jreback @artemyk Can you have a look? 
I should probably also implement it for `read_sql_table`
 @jreback The implementation is not very clean (also some repetition in SQLTable and things like that), but I would like to get this in for 0.15 (functionality is in place, tests pass). And we can clean-up or refactor later if needed.
 @jorisvandenbossche go for it. cleaning is for 0.15.1 !
