App Placeholders

Porting a proposed v2 enhancement to v1.  Here are the updates so far:
* fixed duplicate sourcemap call in Gruntfile
* added F2.loadPlaceholders
* fixed comments surrounding the CSS classes that are applied to apps
when rendering
* fixed a bug where the wrong CSS classes were being applied with the
default AppHandlers
* added logic to allow placeholder apps to bypass AppHandlers
* added autoload logic for placeholders
* added a basic container to demonstrate placeholder functionality
* added unit tests surrounding placeholders
* fixed a bug in the example containers where the proper CSS classes
were not being applied
* fixed failing unit test related to `F2.removeApp()` that appears unrelated to these updates

Remaining To Do:
- [ ] When using the "autoload" there is no way to customize the ContainerConfig.  Should there be?
- [x] Currently an ID selector is used for determining "autoload".  Should a class or attribute be used instead?
* classes and attributes are now supported
- [x] Currently the entire document is scoured to find apps with a `data-f2-appid` attribute.  Should the search be narrowed to elements with an "autoload" attribute?
* only elements with an "autoload" (id/class/attibute) will be processed
- [ ] Currently only the `appId`, `manifestUrl` and `context` properties of the `AppConfig` are pulled from the placeholders.  Should more properties be pulled?
- [ ] Update the docs.openf2.org to explain the feature