Tasktiger

* Stop heartbeat thread in case of unhandled exceptions ([335](https://github.com/closeio/tasktiger/pull/335))

* Heartbeat threading-related fixes with synchronous worker ([333](https://github.com/closeio/tasktiger/pull/333))

* Implement heartbeat with synchronous TaskTiger worker ([331](https://github.com/closeio/tasktiger/pull/331))

* Adding synchronous (non-forking) executor ([319](https://github.com/closeio/tasktiger/pull/319), [320](https://github.com/closeio/tasktiger/pull/320))
* If possible, retry tasks that fail with "execution not found" ([323](https://github.com/closeio/tasktiger/pull/323))
* Option to exit TaskTiger after a certain amount of time ([324](https://github.com/closeio/tasktiger/pull/324))

* Purge errored tasks even if task object is not found ([310](https://github.com/closeio/tasktiger/pull/310))

* Added `current_serialized_func` property to `TaskTiger` object ([296](https://github.com/closeio/tasktiger/pull/296))