Improved filter testing
Value assignment, bug fixes

- Started working on value assignment. Simple assignments using the '=' operator seem to work correctly.
- Added basic tests for value assignment.
- $var.foo.bar syntax is now also possible next to $var["foo"]["bar"]
- Big cleanup in _filter()
  + Refactored the main conditional block
  + Fixed a bug where using sub-expression in selecting object elements would result in incorrect results
  + Removed key range code: was hacky, needs solid implementation.
- Removed avg filter function: can be accomplished using `sum/length`.