CYB-347 Select image fields for Gallery API

https://jira.ets.berkeley.edu/jira/browse/CYB-347
Merge pull request #208 from coyote/sql_get_gallery_data

CYB-347 Select image fields for Gallery API
CYB-348 author_canvas_user_id & created_at should be integers instead of strings

  * numerizes those fields in comments
https://jira.ets.berkeley.edu/jira/browse/CYB-348
Merge pull request #209 from coyote/numerize_comment_fields

CYB-348 author_canvas_user_id & created_at should be integers instead of strings