Add optional graph_def parameter #78

Allows user to enter the graph definition instead of simply using
the metric query. This will make it possible to obtain snapshots
of any query and visual type.
Refactor snapshot from graph_def into a new method
snapshot_ready method to snapshot API and fix failing integration tests
Make handle optional for commenting and fix failing integration test #29
Undo changes that break backward compatibility
Import from urllib.parse in python 3
Update docstring and refactor http request call parameters
Merge pull request #109 from DataDog/isaac/bug_fixes

Isaac/bug fixes