After use in a couple APIs, I found having to check against QueryResult.row and QueryResult.rows too difficult, so this change will always have content in QueryResult.rows, even if it's one row.
In addition, it turns the object into an iterator and adds __repr__ and __len__ magic methods to make interacting with it easier
Even though ``0`` is documented as the max pool size in the aiopg docs, setting it to 0 causes an exception in `aiopg.pool.Pool._fill_free_pool`:
```
File "/Users/gavinr/Source/PSE/anabroker/env/lib/python3.7/site-packages/aiopg/pool.py", line 208, in _fill_free_pool
if override_min and self.size < self.maxsize:
TypeError: '<' not supported between instances of 'int' and 'NoneType'
```
Set to the same default as aiopg, @ 10.
