ExecuteContext should track execution time by default

Fixes #47
Add basic perf and event counters to QueueTask

Use timers in ExecuteContext to implement basic performance counters for
execute duration.  Also, add global counters for basic execution
scenarios (completed, unhandled, trylater).

Partly fixes #50
Deprecate .travis requirements files

Instead, just mark all travis dependencies in setup.py's `tests_require`
Merge branch 'travis'