In-browser profiler: DevTrace

Implement DevTrace for Phoenix so traces can be viewed instantly when developing an app.

Proposed items for an initial release

[x] Handle AJAX requests
[x] Ensure it works w/o entering an API key
[x] Handle sub-millisecond response times in the speed badge
[x] Ensure traces are formatted in proper Elixir format
[x] Possibly reduce the width of the sidebar ... a lot of the trace lines are truncated.
[x] Enable for specific environments
[x] Don't report and log an error with a blank key. See #6.
[x] Update README
[x] Update help site. Mention DevTrace as a way to debug custom instrumentation.

Add backtraces to trace items
Ability to instrument async/await tasks and display in traces
Ability to instrument fire-and-forget tasks + background jobs (ex: exq) and pop these in similar to AJAX requests
Update the DevTrace Landing page so it has Elixir instructions.