Update #inspect, #to_s for App
Extract InternalApp to its own class
Remove trailing whitespace
Extract InternalApp specs for opts
Fix up App fullscreen specs
Use simple spec tagging
Extract #add_child specs to InternalApp
Extract #clear specs for InternalApp

Also, fix dsl app context...the input_block was not actually being
passed to the App object (and by extension, the InternalApp object).
Whatever specs were passing before were not actually testing #clear on
an app with content. Updated specs to verify that there is existing
content before clear.

And also, add #inspect for App, InternalApp
Merge #clear fixes from #762
Edit contexts, docstrings for consistency and clarity
Merge pull request #759 from shoes/internal_app

Extract InternalApp and specs to their own classes

Changelog: bugfix (Although I don't think it happened in this series of commits, #fullscreen now works for OS X)