Add test to detect failure in snapshot API

Currently, there is at least one bug with snapshot API, but
tests are not detecting. This adds a test to detect when event
overlay is present in a snapshot.
Fix bug in requesting event query on snapshots and 2 other things

1. Fix bug in building the request to snapshot without events (Fixes #99)
2. Remove commented code
3. Add .idea to .gitignore
Assert event_query is not in the response and output response as assertion message
Use Pillow instead of pil
Clean up requirements code in setup.py
Merge pull request #104 from DataDog/isaac/snapshot_api

Fixes #99 and improves testing.