simplify middleware registration; dependency on mojito-contextualizer

=== DO NOT MERGE YET ===

- Simplified middleware API registration. Documented changes in HISTORY.md
- Removed mojito-contextualizer middleware, and instead add it as an external dependency
- package.json: updated version to 0.6.0, updated YUI to ~3.10.0, added external dependency and added myself to maintainers list


## Checklist

- [ ] Publish mojito-contextualizer to npm
- [ ] Merge this PR with mojito#next