Set up spec fixture preparation

* Add prepare_fixture and cleanup_fixtures spec helper methods
* Run prepare_fixture before flat view hierarchy app spec
* Run cleanup_fixtures after each app spec
Use prepare_fixture in all app specs
Add live reloading app spec
Reload view registry on each request
Use assets fixture for live reloading view spec

* Test <h2> contents instead of <title>
Add live layout reloading spec
Add live asset reloading spec
Add live asset reloading

* Un-memoize Hyperloop::Applicaion#assets
Reorganize live reloading specs

* Move live asset and view reloading specs into their own contexts
Add spec for live reloading partials
Remove semicolons from asset live reload spec
Add specs for no live reloading in production

* Add set_rack_env and reset_rack_env spec helpers
Disable live asset reloading in production
Fix live view reloading in production spec

* Change contents of <title> instead of <h2>
Disable live view reloading in production
Extract fixture changing to change_fixture helper
Fix comments in production asset app spec
Add examples to change_fixture spec helper docs
Add refactoring note to prepare_fixture helper
Clean up old fixture in prepare_fixture

* Use mock_request spec helper instead of manually creating and reusing
  Rack::MockRequest
* Make mock_request spec helper take an app arg