Revert "[#518] added the missing plugin test"

This reverts commit 89faa535282f350fce58a7fa083553fa20c8c364.
[#515] :rocket: Fix audio-spec in Safari

- Disable tests that are known to fail (e.g. "ogg" format not supported in Safari)
- That's enough to get the end-callback working in Safari -- needs more investigation
- Exposes the internal `codecs` support from Howler. Pushed upstream: https://github.com/goldfire/howler.js/pull/168
[#515] update Howler to v1.1.21
[#515] :boom: Fix audio load and error callbacks

- Howler onload and onloaderror callbacks are synchronous.
- So fix the audioTracks reference before continuing.
- This will solve a lot of debugging headaches later!
Delete unused entity2.js
Better exception handling

- Created a new exception base class; me.Error
- Updated all explicit throws to use the new error class, and derived error classes.
sync;ed with master