whitespace
ViewModel: expect generic Backbone.Model as data model

nstead of requiring an aeris.Model

- Facilitates integration with other libraries and code
- Required refactoring of `getDataAttribute` method,
  which no longer has access to aeris.Model#getAtPath method
util#path: Fix implicitly declared variable
Add edge case test to util#path
Docs: fix whitespace in public docs class descriptions
Docs: minor style change
Marker(Collection): Mark as @publicApi, add usage docs