Fix isolate leak on teardown

[bpcreech]

We were sometimes leaking isolates on shutdown because of reference cycles. In particular, the IsolateManager could be running a perpetual (or self-restarting) task which contains shared_ptr references to objects which in turn contain shared_ptr references to the IsolateManager.

This change simplifies the lifecycle of most things in the C++ side of PyMiniRacer. Instead of using shared_ptr to manage lifecycle, we more explicitly manage lifecycles. Any objects which send tasks to the IsolateManager are, obviously, responsible for ensuring those tasks complete before any objects those tasks reference (usually including the task-creating this object) are destructed. IsolateManager::Run now returns a future to make that easier to do.

Closes #62.