ViewBuffer class to handle circular buffer over data

The view buffer class abstracts out the handling of the circular buffer
logic from the core of LiteList.  This also makes it much easier to
test this portion of the code.

Refactored the tests to separate files.
Fix: Only call shrink if new size < current size

Added a specific check for this rather than automatically falling into
the else clause.

If the requested size is the same, resize simply returns an empty array.
Feature: clear - resets view buffer to empty
Feature: remove - remove an item from the viewBuffer

removes a data item from data, updating all necessary in view indexes.
Refactor to use ViewBuffer

Updated LiteList, RivetsLiteList, demos and tests to incorporate
the new ViewBuffer class.

The resulting code is much cleaner and easier to unit test.

This change did result in a number of breaking changes:
- Height is not exposed in a metrics object rather than directly on items.
  rivets users will need to change the markup to account for this.  See the
  rivets demo for an example.
- The raw data is not exposed as data (e.g. item.data) rather than item.item.
  This should help keep things a bit more clear and better separates the
  circular buffer view from the underlying data array.  See the examples for
  usage.
- LiteList got a major refactor.  The public API is largely the same, but
  anyone relying on the internals (_ prefixed methods and/or properties) will
  likely be broken.
- RivetsLiteList now exposes viewBuffer from LiteList rather than itemsInView
- Tests contain additional refactoring to expect the ViewBuffer changes
- Tests now support source maps to aid in tracking the source of failures.
Feature: added forEachInView to iterate circular buffer
Update rivets dependency to 6.8

6.8 fixes a subscribe/unsubscribe issue
Refactor: use prototypal inheritance rather than aggregation

This simplifies the logic in RVLiteList a bit and is a bit more
standard - less chance of causing confusion (subjective)
Merge pull request #7 from davidcl64/viewbuffer

ViewBuffer - refactor view buffer management into separate module