moves table generation query to payload instead of param

[andy-esch]

in CartoContext.query, you can create a new table on carto, but long queries like you'd have with data observatory augmentation were causing a 414 (too long URI). So the query should be moved to the body of the POST. That's what this handles.

closes #310, closes #335

[coveralls]

Coverage decreased (-0.2%) to 97.561% when pulling fc64f6d on query-post-payload-and-errors into 6c1fddc on master.