Remove unused fallback URLs from app plists
Include non-user-specified defaults

Each handler now has an inherent default app that will be used by
default.
Remove unused INKHandler property
Publicly expose INKHandler's default app
Move core code files into the 'Core' subfolder
Fix fallback URL regression

Caused by two problems:
1. Renaming "Fallback" -> "Defaults"
2. Making the fallback plists a property of the INKBrowserHandler
subspec rather than core.

We should backfill this with a test to ensure defaults work. Also, those
default plists should probably belong to the individual subspecs instead
of browser.
Add availableHandlers method to INKApplicationList
Extract out UIView for displaying a masked app icon
Add 'name' property on INKHandler
Application list can return a specific activity
Expose INKAppIconView's masked image
Make development environment mirror pod usage

In development, the library is now loaded as a Developmet Pod.
Fix bug related to masking INKAppIconViews
Basic defaults view controller
Can set defaults using defaults view controller
Don't show remember toggle when setting defaults
Fix broken callers in tests
Add isAvailableOnDevice property to INKActivity
Fix documentation typo
Prompt to set defaults only shows available applications